Two subs, yes or no?

I notice the use of twin subs often, why is that? Are two subs more of a balance thing? Or better sounding overall?
If you are using two small 'monitors' having two subs will allow you place the subs close enuf to the monitors so that stereo signals coming from the sub (above the nominal cross over point) will integrate with the same sound from the monitor and it will be more seamless.

If you are using the subs to reinforce or supplement low frequencies of floor standers using two subs will allow you more freedom in placement in the room to avoid large nodes. You run the mains full range and then adjust the output level and cross over frequency, phase, etc, to match depending on where you put them in the room. You can usually do a pretty good job with just one, but often two just works better.

If a sub is operating at its intended range, you should really only need two when the output from one is not enough. Subs should be crossed over as low as possible, and I believe anything below around 80 Hz is omnidirectional. Meaning, humans can't tell where the sound is coming from. (Aside from placement issues)
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Bob is correct. 2 is better than one. With 2 mono subs you will be able to minimize room effects, esp. if located asymmetrically in the room. There are also advocates of stereo subs for music, there is only a mono LFE track for HT. If you are going with a stereo pair, I would locate them near your mains, but still asymmetrically (one inside left main, one outside right main, for example).
HMM, mono you say..

Let me throw this at you. I have learned I can use my second set of outputs/rcas from the variable side of the mcd201 direct to a sub. If two subs are run in mono does that mean 1 rca (+), to sub 1, and the other 1 rca (-), to the second sub? Does that equate to a mono hook up?

Or do you suggest sticking to the binding posts of the amp for connection, in mono.....
If the pre-amp has variable line outs, there will be a left and a right. In that case, if you run one direct to each of a pair of subs, they would be a stereo pair. However, the pix of the rear apron of your mcd201 here ( shows one set of variable RCA outs and one set of fixed. Without an active x-over of some kind, you would not be able to level match the subs to the mains unless the XLRs were active at the same time and either your sub or your amp accepted XLR inputs. Same answer as far as hook-up, tho. You would connect right output (xlr or rca) to right sub and left to left. Alternatively, you can take the full range signal from your main amp and either route it through a sub(s) (with a high pass filter) to your mains, or parallel the sub(s) with the mains by running speaker cables to the sub(s) and the mains from the taps on the amp. I admit, its confusing. If your main use is HT, and if the output (volume) of the LFE track is sufficent, then I would go with a single, higher quality sub. If your use is mainly music, I would go with 2 subs if your budget can take it. Stereo or mono, depending on the equipment you are using.
Hey thanks, and to clarify, the mcd201 can run balanced to my amp (my preference), and rcas to sub(s) simultaneously...
Music 100%, no HT, budget would allow 2 x Velodyne SPL1000's..
Overwhelming yes! Two subs fill up your space faster, and as bass waves move slower, this will make the bass seem lighter and faster, even if you went with cheaper subs. The trick will be dialing them in so that they mesh well with your mains and then making them work in your room so they aren't just "one note" bass boxes.
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Hey thanks guys, this is very very helpful.

I was at my favorite dealer Ovation Audio this weekend and Mike and Rickey spoke of running 2 x mono to integrate with my xls360's..
Two things as I was not clear. When I make reference to using both rcs and balaced output at the same time, in fact it is rca and balanced out of the variable output.

The other thing is Swampwalker suggests the position as asymmetrically (one inside left main, one outside right main, for example), which is weird because this very suggestion solves my left corner where there is a door to my balcony! Problem solved.

You will have to experiment for sure, but the idea of asymmetrical placment is to (theoretically) help with room modes. If both the balanced and RCA variable outs are active, that will help, but you may have to compensate for differential gain via se and balanced circuitry IF it is truly balanced and not just an XLR connector wired to se circuit. If not, then the differential gain issue disappears, and could be handled of course when adjusting sub gain to match mains.
Just my 2 cents.. If you can track down a pair of Rocket UFW-10 subs you'd be very happy.. Best small sub I've ever heard, I picked one up a couple years ago (only needed one for the size room we have) and I couldn't be happier. Incredible performance for a smaller sub, if your mostly into music it's just perfect, it's quick, tight and can go deep and as far with HT I have absolutely no complaints, does a great job. I still have some of the reviews bookmarked I'll post here, only problem is I see they no longer build this sub, not sure why for it was one of they're biggest sellers.. I have the Rosewood finish model, has a glossy Piano black top and the included brass spikes with discs under them(hardwood floor protection) only reason I mention the finish & feet is because I have this sub in our main living room and it blends in nicely, not like most subs that look like a black box in the corner.. Actually my wife has a tall artificial Fiscus Tree on top of it, believe me if looked bad or out of place she wouldn't be shy letting me know :)..