dual subs.....how & where to install?

Ive just added another revel b15 to my existing b15.

Does anyone have experience of where is the best place to install the second unit? Ive always found with one unit, the room is pressurized from one side of the room allowing localisation of the sub, even though the x-over is set at 60hz.

I planned on putting the second sub on the other side of the room, one sub by each revel studio speaker, however someone mnetioned stacking subs on top of each other.

Any ideas gratefully appreciated,
My subs sound their best, (most musical)....behind, and just to the inside of my speakers. My room is 25' wide...this places the subs at around the 1/3rd room width points on that wall with the fronts at 24" out.

I also got good bass levels in the front room corners...not musical, even though that placement did measure fairly well.

I also tried the two side walls...not as good as the front wall in my room.... also, not as musical sounding.

My mains are planers and I'm "very" happy with the locations I'm at now...It's been truly a game of inches.

This is a highly debated subject. You are correct, most subs are detectable even at low frequency. Hearing is not the only thing at play here. First off I would suggest lowering your cut off frequency down to 30Hz since your Studios are 3db down at 33Hz. Localization is very easy on a sub with a 60hz cut off anyway. Set at 60hz you probably have too much bass in the 30-60hz range.

You can try the stero sub set-up i.e. one near each speaker. Effectivly, you are creating fuller range stereo speakers with the two subs. The drawback can be cancellation of certain frequency's due to the sub spacing. Think about it like dropping two stones in the water some distance apart. the waves colide and some cancel while others add. The stereo effect is difficult to distiguish as the frequency goes down, so low frequency tends to sound mono. Stereo seperation becomes less important and sound wave cancellation can become an issue. Stacking subs is like throwing one bigger stone in the water. There is probably not much stereo information below 30hz so you may find it gives you better impact. You will need to experiment with your room and set-up to see what works best for you.
I'm aware of at least two schools of thought on the subject.

The first would recommend placing one sub to the extreme right of the listening position, and the other to the extreme left of the listening position. This works with the ear/brain system to produce the greatest sense of hall ambience.

The second school of thought would recommend placing the subs as asymmetrically as possible, so as to maximally stagger the path lengths from each sub to room boundaries and then on to your ears, resulting in smoother bass over a large area. A low crossover frequency and/or steep slope low pass filter is beneficial in this application. This is the technique I use. In my room one sub is behind the right speaker and near a corner, and the other is along the left wall slightly behind (and much closer to) the listening position. Ideally, one of the subs would be elevated off the floor also.

I would agree with Audiokinesis. It really depends on what you are trying to accomplish with the subs. In a asymmetrical configuration i have in the past set up one in the front of the room with more audible bass. In the rear i set it at a much lower freq. How big is your room?
Setting up two subs is very difficult to do well by just using yor ears. I'd recommend getting something that will give visual feedback such as a velodyne sms-1 or a measurement program like ETF.
My two cents....place them at the same distance to your listening position as the speakers....with two full range speakers you have a lot of drivers covering the 20 to 90 Hz range (even after filters)

Therefore place one on the left of the left speaker and slightly in front along the wall (maintaining equal distance to the listening position as the two speakers). The second you can do the opposite.....right of the right speaker, slightly in front and along the right wall.

As Duke says, there are two schools of thought......I have tried both (but only with one sub);

My impressions are:

1) If you seek smoothest sound by locating the sub in any old position there will be inteference between the sub and speakers on the primary signal, as the sound waves are not aligned at all frequencies. This makes the room reverb better but it destroys the clarity of the primary signal. The bass sound loses impact and is slightly disconnected from the music. ( A solution for this is to use the THX rule of cutting your main speakers at 80 Hz and using the sub for signals below that....again I find this disconnects the bass from the rest of the sound as there is plenty of energy in the 50 to 80 Hz region despite the filtering)

2) If you seek alignment of the sound with the main speakers by placing the subs in front of the listener at the same distance as the main speakers then the sound will be tighter and more integrated for the primary signal....to me this sounds best, despite room modes being noticably more excited in this configuration ( I use a PEQ to take the edge of room modes by slightly reducing the sub signal at the bumps)

The above are my observations after much experimentation. I can't say that I can locate the sub except when I am within a few feet and it is obvious where the high pressure comes from, however, I do sense when the primary bass sound waves are aligned and integrated at the listening position. The sound becomes tighter and cleaner and the "room sound" is also correct(but may be more intrusive).

By "room sound" I mean that the reverberation in the bass now matches the rest of the audio reverberation signals...I suspect our brains are able to distinguish primary from reverb signals and one can sense when the sound is aligned for the primary signal (aligned = coming from same direction) ....this may explain how we can so clearly hear someone at our table in a large dining hall with tons of people talking,