why are two subs harder to place than full range?

sorry if this is a dumb question, but i notice that whenever someone talks about getting stereo subs there usually ensues much hand wringing over placement and room modes, etc., but this rarely is the case when someone talks about full range speakers with large 10 or 12 inch woofers (with or without built in amps for them).

Is there a sound technical basis for this different treatment or is it merely convenience, i.e., since subs can be moved about separately from main speakers, then it makes more since to talk about eliminating room modes by moving them around?

I'm getting a second sub for a rather large vaulted room, have an injured back, and would rather minimize what seems like a rather long process, at least for the time being by just keeping the subs with the mains.

The issues are the same for full range speakers or subs. The problem with full range speakers is you often have fewer options regarding placement to optimize bass response whereas with two subs (assuming you have crossovers that roll off the main speakers bass response) you have much more flexibility in placement. So if you have more flexibility, doesn't it follow that you would have more hand wringing (angst)?
Sub-frequencies are simply harder to control than mid and high frequencies. And if a subwoofer is "misplaced" there will be a "hole" in one or more listening positions. Least expensive method of finding optimal subwoofer placement is to place the actual sub at the listen position, (such as your favorite listening recliner - GASP!) & run a bass heavy track or signal while listening for the best bass response from prospective positions in the room where you might want to place the sub. Time consuming, yes. But well worth the effort if you do not have auto eq & measuring mic.
If you Google "Richard Hardesty" and "Vandersteen" you should find soem well written articles on the theories behind subwoofer placement, and why separate subwoofers make for much better bass performance with optimal midrange in the main speakers - the location for both are often not the same. If I reacall correctly, two subs are much better than one for a variety of reasons - not having to do with "deeper" bass but more linear bass perfomance.
Vandersteen says put them in the corners
I'm getting a second sub for a rather large vaulted room, have an injured back, and would rather minimize what seems like a rather long process, at least for the time being by just keeping the subs with the mains.

The best spot is often away from the corner and close to the side wall and a few feet in front of your main speakers (assuming you have full range) I find the best way to integrate is to run the main speakers full range if they are big bad boys with big sub sized woofers themselves.

I would never place a sub or main speakers in the corner unless you want pure impact for HT - as you are talking room modes galore...
Although I'm only using one sub I did quite a bit of experimenting with placement. The corner placement was the worst for me. My sub is between the speakers with the driver on the same plane. By far the smoothest integration.
The sub brings almost as many problems as it solves. To give you peace of mind connect the system to a room correction device. Some not only measure and EQ to compensate for the acoustics, but will act as a crossover too.

Some are simpler than others to use. I can recommend the Lyngdorf room perfect system for its ease of set up and excellent sonics. It will save a lot of fiddling about. Speaker placement is then irrelevant.
Good subwoofers belong in the corners of your room so they can properly load it. Your mains belong out further into the room to avoid boundary effects(study 'The Allison Effect'). The problem then is the time alignment between the subs and the mains. The better active subs have time and phase adjustments that enable one to align the system in the time domain, or a unit such as the TacT RCS 2.2XP or Lyngdorf Room Perfect System can be used. Here's something that may interest you: (http://www.linkwitzlab.com/frontiers_5.htm)
the problem is not one of subwoofer placement; separate subwoofers are easier to place correctly than full range speakers. the problem is integration. unless the subs perfectly integrate with the 'other' speakers seamlessly, perfect placement has limited value.

we are speaking here of 2-channel music only listening......HT DSP LFE channel subwoofers are a different issue.

the problem is somehow getting the crossover for both the sub and the main speaker to work as if they were designed as one unit. also to have the character of the main speaker and amp to be captured by the sub. most normal speakers do not have a low frequency cutoff or way to vary the low frequency extension. therefore you generally need an outboard crossover; which typically reduces refinement and adds complexity (or digitizes everything....bad, bad, bad)

lots of people do run separate subs and like them; i've yet to hear a system like that integrate to my likeing.....but i suppose it is possible. note i am not saying that i don't like systems with separate subs; only that they lack the level of mid-range--mid-bass integration the very best systems have.

i prefer built-in subs which ARE designed into my speakers and give me 10hz extension AND seamless integration. i don't have as much flexibility in location; but i have lots of subwoofer adjustments to help the bass integrate with the room.

integration.....not location....is the real challenge.
thanks everyone, your responses helped alot, sorry i'm just responding but have been away all day and night. thanks again everyone.
Actually- especially for the novice - I wholeheartedly recommend keeping the subs up near the main speakers, especially in systems with adequate room DSP and/or Parametric EQ's, ideally. In these cases, it is most often much easier to get a coherent sound, seat to seat, with subs placed up near the mains - I find.
Yes, you need to consider the output capabilities of the subs and the volume of the room. If your sub/volume ratio is on the short side, you might consider close corner proximity for the subs, with some play. Still, if they have the output, and you can "EQ" out the response of the subs somehow, you have the strong advantage of not running into as many PHASE issues between where the mains leave off and the subwoofers pick up - among other things. And, less than ideal "modal placement issues" can be EQ'd out, as long as you're not setting the subs in "holes" in the response curve.
Basically, I like the blending ability and coherence potential of subs near mains, personally.
If you put one subwoofer next to your seat, sidewall, etc, and you have multiple seats aroudn the room, you are most assuredly running into phase issues from speakers, to subwoofers, to seating possition, ideally. I don't like this tradeoff personally. Also, If you start putting subs at 1/4 points around the room, you are DEFINILTEY starting to deal with "phase cancelation" and comb-filtering issues, most often. And this is also hard to deal with, if you're not careful and know what you're doing.
My rule here is to try and "keep it simple &*#$@%!" That way you have a better chance of well balanced, dynamic, coherent sound.
I once did a system for a guy who had a couple of monster high end subwoofers, set up mostly out of phase with each other, and the mains! He complained of "no bass" in the system, even with the subs cranked up. I placed his subs up near the mains, right next to each other, off center of the room slightly - and it sounded great, but he didn't like the look. We ended up with mostly the same performance with one sub out of phase, placed in the corner, to good effect.
Hope this helps
You can EQ a room to death and still have bad sound.
Take the case of a room with dimensions which are even multiples of one another.....say 8foot ceiling with 16x24 foot floor size. NO amount of EQ will get you away from all sorts of standing wave problems.
HSU (Dr.) recommends near-field sub placement. Right next to you, if possible!
The Harman White Paper recommends 2 subs, placed assymetrically. If anyone is interested, I'll find the link.
My personal listening space is a very odd shape. 8 walls, some at a 45, and a vault ceiling, also not symetric. My sub is near the mains, up against a side wall (the longest wall) and about 4 feet from the corner. Downfiring sub, with the port facing backwards. Decent integration, and with the X-over at 40hz, quite seamless with my Maggies. The room is well over 5000ft3. This is the right way to measure a room, since for any given floor space 8' vs 12' vs high vault ceilings will make quite a difference.
iplaynaked offers the best first cut for my situation, and i appreciate the advice. if that doesn't seem to work i'll wait for my back to heal and start moving some furniture and listening to the sub in different positions. btw. my room is about 14'x 23' with 12' vaulted ceilings. i'll be using a pair of tekton design fostex 167e single driver speakers on the short wall and two cheap aural ecstasy subs (i expect not to turn them up very much) listening at approximately 9 feet away. thanks again everyone.

magfan, in my last apartment that is almost exactly where i had my, at the time only one sub, and in the much smaller room i had huge bass with almost no volume (xover @ 60hz) and what seemed good integration (at least it didn't seem to take away from the sound of the tektons running full range, just an addition of lower frequencies).
First get a dolly so you can easily roll it around. In most rooms, deep bass does not couple in the same plain as the mids/highs. If you notice, most really expensive speaker systems have separate subs and a XO

If you use one sub, try to place it on the right side. NOT IN A CORNER.

The wringing of the hands is worth it

Whatever works in THAT particular room.
/ listen / move sub / listen some more.
repeat until satisfied. Than leave it alone
for a while.
btw, here is something some of you may not already know concerning the use of subs with t-amps http://www.audio-magus.com/help_answer.asp?ID=20#135

although i didn't know this and connected speaker level anyway, my amp seems to have not suffered damage.
Generally fullrange speakers offer fewer placement options than two subs would, but on the other hand the designer probably has a pretty good idea of where they will be placed in the room. With dual subs, the range of possible placement options expands enormously as compared to a pair of fullrange speakers.

If you can crossover at or below 80 Hz, and with a 4th order lowpass filter on the subwoofers, then you have a great deal of flexibility regarding subwoofer placement. If you have to cross over higher and/or do not have a steep lowpass filter, then keep the subs near the main speakers. The point of this paragraph is, you do not want to be able to hear the subs as separate sound sources due to lower midrange energy coming through them.

Assuming you can place the subs as you see fit, yes there are several different placement strategies. I favor asymmetrical scattering to average out the room-induced peak-and-dip patterns generated by each low frequency source. You want the subs to be spread out in at least two dimensions, relative to the room boundaries. As the number of spread-out low frequency sources goes up, the importance of the exact placement of any one of them goes down. So in practice, multisubs are usually more placement-forgiving than a single subwoofer is.

thanks duke, i really respect your postings, both in terms of tone and content.
Try this link on for size.
Some people have already said similar stuff, but this wraps it up and presents it properly::

For more than a dozen years I used a pair of Magnepan MMGs, each sitting atop its own PSB Alpha subsonic. The combination looked beautiful, and sounded wonderful. I positioned them where the MMG's sounded best, and the subsonics always followed gracefully.
I'll bet my 1.6s would scare you sitting on top of my HSU VTF2mkIII. Total of about 7 1/2 feet tall!