Best setup for two subwoofers.

FWIW,

My own experience would suggest a few things, all of which have already been mentioned, but none of which may be clear from the debate:

If you lack bass management software in your system, an array of four optimally placed subs is likely to provide significantly smoother bass response at your listening position than an optimally placed single or dual subwoofer set-up. If you DO use bass management, the delta in performance between more subs and fewer subs at your primary listening position is likely to be smaller. IME (and I did try a decidedly poorly controlled A/B comparison at one point), a single (or pair of) bass managed sub(s) may well provide smoother response than a quad array of non-managed subs at the primary listening position.

However, you may still have smoother response over a large listening area with a "non-managed" quad subwoofer array vs a pair of managed subs - since bass management software IME optimizes response in a smallish window. As to optimal placement of such a quad array, you're on your own. Expert opinion seems to vary, so - if you end up with four subs - you should probably experiment with both in-corner and non-corner subwoofer placement to determine which sounds better.

As a practical matter, for a 2 channel, music only system, bass management software is still rare. In this case a quad array is likely to be the better choice. Once you introduce bass management software, the incremental value of additional subs is less dramatic. Of course, everyone has their own particular sweet spot on the curve of diminishing returns.

Since you're using an HT system, it's likely that you already have bass management software. If that is the case, the primary advantage of adding additional subs is the incremental gain in clean output capability at any given frequency. Even the best subwoofers' distortion increases dramatically with SPL at very low frequencies (tho the best models offer mighty impressive performance in this regard). A single monster - like the 1812 - may provide more than enough output for you. Still, two 1812s will get you more. So, in this respect, (assuming the use of identical subwoofers) more subs are better than fewer subs. But IMO, the real question here is.... overkill. How much clean output capability do you really need?

Well...,

How loud do you listen?
How big is your room?
How deep is the is the lowest frequency you care about reproducing cleanly?

IME, I'd say that a pair of high quality, high output 12" subs (SVS, Rythmik, HSU, etc) probably does the trick for most listeners in most HT rooms that have bass management systems. However, it's just not possible to say with certainty that it will be enough for you. You can always start there and add more if the need arises.

Good Luck,

Marty

Don't know if this is off-topic but what opinions might the posters' have about a set up like the Paradigm SUB-1 or SUB-2 instead of multiple subs in separate enclosures; that is, multiple drivers facing different directions in the same cabinet? The Paradigm subs have 2 ea drivers on three surfaces (facing different directions by 120 degrees). Would this arrangement provide some small measure of an effect similar to two (or more) subs oriented differently (as described in this thread)?

Thoughts?

Full disclosure - I have a SUB-1, Maggies, and a Meridian g61r; the Maggies and SUB run "full range" with the sub's crossover at about 50Hz. I ran the Meridian room correction without the sub, then added the sub and ran the Paradigm PBK SW. The net result in my room as confirmed by over a half-dozen "golden ears" is that my room has a very, very uniform bass response. ghl

You're dealing there with a matter of likely increased efficiency, with more drivers or volume, power, etc. And, since the woofers are in pretty much the same acoustic location, it acts acoustically like a normal large subwoofer, in space. So, whether the output, freq depth, and ability to fill volume in a space is right with any given sub, in any given space, is a key there. OTher than that, how quality is the actual product and sub performance on it's own...is it accurate and fast enough as a music sub to integrate with the system...does it hold together and make an effective home theater dynamic subwoofer, capable of reproducing explosions and heavy dynamic loads, etc?? These things are obvious considerations.
If all of the above are covered well with single subs in a system/room, then the question of multiple individual subwofers is more relevant for addressing issues like more needed bass volume/energy/output in a given space, getting more efficiency from the system (more muscle from the bass), and helping the systems bass acoustics, by trying to more evenly smooth out he bass modes, which are inevitable in most any practical size home theater environment. In case of the last issue, I think it's been proven that carefully place multiple subwoofers in a room, has some key acoustical benefits/advantages, that are worth noting.
My problem with multiple subs placed anywhere other than in either one location up near the front of the system, or flanking as stereo subs near main speakers, is that of PHASE ISSUES, from multiple seating locations, in relation to mono subwoofers at different distances from the listeners ears!!! And I've proven that this can be a BIG challenge to set up right, particularly in multiple seating arrangement setups! (once had a client who had two mega buck subs setup in different locations in a room, and the subs weren't making any noticible bass impact across the spectrum! The problem?..they were out of phase with each other, and at varying seating location distances, in relation to the speakers and the subs themselves! -an EASY problem to run into, I assure you - relocated the subs up front, fixed the problems, and super depth, impact, coherence, speed and effectiveness! problem solved, thx)
I MUCH favor keeping the subs in an inherent good singular location in the room, preferably up front (where crossover blends are MUCH MUCH easier), and where I can EQ the subs issues out then, and keep the phase and coherence of the system intact, regardless of the planned seating locations chosen. I think the vast majority of audio enthusiests would find this approach a much easier setup approach, with more foolproof results, and less chance for problem - better matching and blending, and so on.
Then again, if anyone wants to tell me how they can avoid having phase cancelation issues between two(or more) mono subwoofers, placed in half spaces of the room boundaries (sides, front, back, etc), with several rows of seating locations (or even sitting right next to one side sub, vs the one in the center front), in relation to other seats, and still not have phase cancelation issues, and lack of coherence between bass crossover and main speakers crossover frequencies, etc, then I'd sure like to know about! Cause I find nothing but problems with the multiple mono subs placed midwalls, all around the room, unless dealing with 1 or two seating locations only. And even then, nothing a good DSP EQ can fix on it's own, anyway...so what's the point, I've always asked?
Much rather do multiple subs around the room, near each monitor, run in stereo individual channel configuration for each speaker in stead, in that case.