Go for sub or larger speakers?

What is your experience with sub integration, is it really as hard as they say and the success is far from guaranteed? Am I better off going for larger model if I want more of low end?
Specifically, should I go with ATC SCM50ASL and subs, or ATC SCM100ASL?
This is not for home theather, just two channel system.
I just upgraded to REL B3, and I can't believe how well it integrates with Harbeth C7ES3s.
A lot depends on room size, I think it is easier to get a speaker/sub combination to work in a smaller room. But in the end it is a question of personal taste. I believe most of the integration problems arise from crossing too high or playing the subs too loud or a combination of the two. I haven't had much trouble integrating mine.
Thanks for info, one more question, would I need to employ filters on both main speakers (highpass) and sub (lowpass) for proper integration, or I can leave mains as they are and only filter higher frequencies on sub?
My speakers are 6dB down at 40Hz, I was thinking of connecting subs to 2nd output on pre-amp and employ filter on sub only, in order to avoid additional signal processing for main speakers.
There are two schools of thought on filtering. One believes in using an electronic crossover and rolling off the main speakers. This undoubtably allows the system to play louder. The other, of which I am a member, believes in running the main speakers full range and using the subs to supplement the bass. This avoids the addition signal processing you mention. Subs like the REL, which I also use, can be driven from the amp outputs as well as line level, this allows them to track closely the behavior of the main speakers. I start rolling my subs off at 25Hz on the top end, when I set the level correctly you can't tell they are on. Just adds body to the sound; does this even if no low bass is present.
In most cases, adding subs will cost substantially less than going a model up with main speakers, and get you better bass. Also, since the subs are typically active, you are giving you amp a break with the subs.

What I did with absolutely stellar results is this. Get 2 very fast subs (I have JL Audio 113). Run 2nd output from your preamp into an external crossover (I use the cheap NHT X2). run the low pass signal into a SVS (or audessey - some unit), 2 channel sub equalzer in discrete stereo mode. Dont mess with your mains signal - run them at full range. Play around and do some in room measuremt to find the best crossover point for your low pass filter. Apply bass EQ to your stereo subs.
I am with Stanwal for the most part, though I haven't rolled any sub off as low as 25 HZ.

To me the integration is the different speed characteristics. A "slow" sub is very difficult to mate with speakers. This is the area that is most noticeable as well in listening to music.

I am suspicious when I see subs with 15" drivers advertised (especially if they don't cost a fortune $5K-$10K+, and to some degree even with those that do cost a fortune). A 15" driver is a lot of driver to move quickly and stop quickly.

In my book, the faster sub will be easier to integrate. I would rather give up some absolute bottom frequency extension to insure I get the speed I need (especially with your ATC speakers).
Ckoffend - you are dead on. I had 15" Revel B15s and the 13.5" JL 113 in the same room, and although the Revels go somewhat deeper, the JL's are faster and easier to integrate.
For 2-channel audio, I prefer to avoid a sub. I'd rather that a top-rung speaker designer design my crossovers, than me.
I use the hardware Edorr identified, but in a different configuration. I filter the main signal with the NHT X2 and run the bass signal out of the X2 into a Velodyne SMS-1 room EQ unit, because:

My current room, like every other room I've measured, gets very ugly below app. 125hz. I can (and do) fix it down to app 75 hz with bass busters. Below that, active bass EQ is the only workable solution that I've found.

If you run main speakers that have any real output below 75hz, it will IME be very difficult to get seamless integration of the subs and mains because the main speakers are contributing very lumpy bass until their roll-off. If you filter the mains to remove bass below 75hz (or at whatever frequency your treated room dictates) you can get smooth response from the mains, smooth response from the EQ'd subs, and a neat hand-off that will hard/impossible to hear (for me anyway).

If your main speakers have little to no output below 80, 90, 100hz, (i.e. Sunfire cinema Ribbon Monitors)you can probably run them full range and "snug up" the subs from below. Otherwise, I prefer to filter the mains.

As Stanwal, noted, there are 2 schools of thought, and this is the school I'm enrolled in. Others take the alternate route.

Two more thoughts:

1) "Fast" is a descriptor I dislike for subs. People tend to think small drivers are fast, and - IMHO- that's not the case. I think "highly damped" probably communicates the idea at hand - tight bass with little overhang - but this characteristic is not related to driver size.

2) My set-up does put the NHT between my ARC LS-25 and my ARC 130SE, but it keeps the Velodyne SMS-1 out of the main signal path. The NHT seems to be benign (to my ear at least) but the Velo has no business in the main path.

Martykl, I'm sure in some systems your approach will work better than running the mains at full range. Its a trade of between smoother integration and the degradation of running the mains signal through an additional active filter. If I had gone down this path, I would personally not have used the NHTX2, but get a higher grade crossover like a Marchand XM44.

This may be pure snobbery because I have no proof whatsoever an XM44 sounds better than the cheap NHT, but I would not like the idea of running my main signal through a cheap crossover when I have some very high grade electronics upstream and downstream. (Sort of the "weakest link" theory). Your appear to be owning some fairly expensive kit yourself, so I would definitely try out a higher grade crossover sometime and see if you get any improvement.
Hi Sashav,
Alot of opinions here. My experience says, if you like the sound of your smaller speakers and your amp will power them properly for the room your are in, you are fine and can achieve wonderful results with a subwoofer. You can run into a problem crossing. If you do not us an electronic crossover to cross your satellite speakers where you want them to blend with your subwoofer, you will need to cross your sub woofer around the 3 to 6db downpoint of your satellites I use a subwoofer now and cross it @ 40hz (its lowest setting). My speakers are 3db down at 38, I set the woofer low and slowly added until it was smooth, played a bit with the phase switch, had to move the woofer a couple of times, but ended up with a very smooth transistion.
As far as woofer size... on a properly designed woofer, size does not matter. A designer should have more motor to match the mass. Good 15's are fast, but you should try to listen to the sub that you want first and consider your room size, an 8, even a 10 would not move enough air for my room.
On the other hand, if you really like the larger speakers.... your done.
To sub, or not to sub? I'm with the "depends on the room" contingent. In some rooms you just cant get "fullrange" speakers to work (without correction).

One more thing you might want to consider: if your main speakers are ported, you may want to plug the ports. In my experience, if your mains go relatively low it can sometimes help integration with the sub. On smaller speakers I usually leave them open. Lots of variables to play with, all rooms and set-ups differ.
Post removed 

I've definitely thought about the x-over issue. I borrowed a (used) Marchand from a local dealer for comparison and it worked well, but it was noisy, which was more intrusive than the NHT. Of course, who knows whether that was just due to the condition of that one used example.

The main reason I haven't pursued it is that I can't decide whether to go for a high end HT pre/pro solution or a higher "pedigree" stand alone x-over. I'm kind of embarassed to admit that my power up regime -1)preamp, 2)SMS-1, 3) NHT, 4) DAC, 5) power amp has me thinking that a pre-pro (Theta?) replacing #s 1 thru 4 might be nice. Not sure I'm ready to punt and go that way, but I'm not sure that I'm not ready, either.

But, your point is definitely taken.

I see that the thread has run in the direction of sub integration. I found that this was actually difficult for all the caveats which appear as aside or minor conditions to be aware of.
You must count the following; room difficulties, with no absolute answer. This is reasonable because no two rooms are alike, so it's reiteritive or back and forth experimentation big time. This can be a source of uncertainty and frustration.
The appropriate frequency to roll off the sub activity, Stanwal suggests 25 cycles. This means only the sub will be playing the lowest 5Hz at the signal strength before being diminished at a non specified rate. To me this hardly seems worth it as virtually no music exists at such depth. To Stanwal's credit he says it is unoticible except for bringing fullness to the sound. So you have to guess at integrating your sub/s at the imagined low frequency where your regular woofers fail to suffice. Let's say anything below 60-80 or so cycles, however integration at this level can sound bad and was hard to do in my own experience.
The use of external crossovers which of course means more signal manipulation and depending on where you put it potentially with the mid and high freq also.
The scheme of ext xover to the sub equalizer and of course the sub's requirement for an equalizer.
The sub speed matching and integrating with you speakers response.
I am sure you will find more if you read carefully.
This is a long winded way of saying I agree leave it up to the Speaker engineers and buy a speaker with sufficient low end to do justice to the music. The music you actually play not the sound effects many subs are meant for and you specifically don't want it for.
Good luck

You could pick up a used Theta Casablance III with 1 Xtreme card for about $4,000, and use it in a 2.1 application. I own a CBIII with XTreme card so I can give you some advice here. From what I gather you want to consolidate boxes, and need four things; Preamp, DAC, EQ and Xover. The CBIII will not give you EQ, so you still need yout SMS-1. The Xtreme card is a good (10 year old) DAC in a MCH application, but a modern 2 channel DAC beats it hands down (I have a PS Audio Perfect Wave which is a better DAC). Xover flexibility is second to none in the CBIII - no issues here. However, your LS25 is a far better preamp than the CBIII (I use a Theta Six Shooter for preamp, which is far better than the analog bypass in the CBIII). In summary, depending on what DAC you are currently using, you will very likely take a big sound quality hit with a CBIII, still have a two box solution, all for the questionable benefit of having a good Xover in a box 40lbs box (with I bunch of features you don't need).

A far more promising alternative is to look into the TACT 2.2 digital preamps with build in EQ and Xover. This has all the four functions you need in one box. I suspect the (optional) buld in DAC is the weakest link - but not to worry, it has digital out so you can hook it up to an External DAC. The only wildcard here is you are going from volume control with your LS25 tube preamp, to doing it in the digital domain. This would be anathema for purists, but since you are considering a CBIII and are not loosing any sleep over using a cheap Xover in an otherwise high grade system, obviously you are not a purist (I say this as a positive).

Thanks for the advice. I appreciate you taking the time to share your experience since it is precisely on point. I've thought about the TacT option and my main concern until now has been set-up complexity (I've seen a number of posts on various forums that have complained about that). However, that concern is now secondary to the volume control issue, mostly because it didn't occur to me that the volume control was in the digital domain until reading your post. One step forward, two steps back....

The likely result is that I will continue to delay any decision and enjoy the system as it is right now, which is extremely satisfyiong top listen to, if just slightly philosophically bothersome to the audiophile in me.

Thanks again,

I have some design experience with large full-range speakers and roughly comparable satellite/subwoofer systems, and imho done correctly the latter will have better bass quality and no audible integration anomalies.

Well, until a week ago I can absolutely say I was totally against using a sub with a full range loudspeaker. I have Avalon Eidolon Diamonds that I absolutely love. The bass is fast and tuneful.

I've heard these speakers in a similar sized room to mine but on a concrete slab floor. My room is an upstairs room with a wooden floor. In my room, with fast and tuneful bass, I have extension only down to around 35Hz. It's just the way the room is and interacts with my Eidolons.

A friend of mine works at a high end AV company in town and they were demoing a REL Gibraltor G1 sub. He suggested I try it out for kicks. Well needless to say my jaw dropped at what I was missing. The REL integrated perfectly with my Eidolons. The ass remained fast and tuneful but with extension down to 20Hz and basically a flat response from 20-40Hz, the sound as amazing.
I still remained perplexed on how changing my sound in the 20-40Hz range gives me tons more air, and widens the soundstage dramatically. Imaging seemed even more solid. And bass impact became a true visceral experience.

I highly recommend people with full range speakers not getting the bass they ant in their particular listening room to try out a sub. You may be surprised. And the way, the new G1 sub from REL is possibly their bet yet. I just ordered one.