For those with dual subwoofers placed at the same distance to your couch, do you set the phase for both of them at zero?
Here's a guide from Brian (Rythmik):
How to set up multiple subs?
One cool feature of phase/delay control on the plate amp is it allows you to compensate the phase difference arise from difference in physical distance. To use this feature, one needs to use line in, instead of LFE in. The purpose is to make sure that direct sound multiple subs arrive at the listening seat at the same time so there is no cancellation in sound. To do that, use the farthest sub as the reference. Then measure the physical distance difference in ft. Note the sound speed is about 1 ft per milli-second. As an example, say the difference is 5ft, then set the phase/delay knob on the closer sub at 5ms. Then one can use room EQ program to calibrate both subs at the same time.
Here's a feedback from forumer neutro:
I finally put my subs in a symmetric position each side of the TV, so equidistant from the main listening location. Strangely I found that using 180 deg. phase difference between the subs both sounded the best and managed to get rid of a nasty null in the crossover region. So phase sometimes have unexpected consequences. The best is to try phase difference from 0 to 180 degs to see what sounds better, simply. In theory, with equidistant subs, setting one at 0 degrees and the other at 180 will create a dipole, and the listener will be exactly on the dipole's null boundary (perfect cancellation). In practice it's not necessarily the case. First, woofers can be wired in opposite polarity in the two subs -- which would require setting the phase to 180 degrees immediately (or changing the woofer's polarity). Second, this is only true in an anechoic setting. Inside a room, interactions with the room will create standing waves and room modes will be dominating the frequency response. What reaches the listener is a combination of sound directly emitted from the subs and sound reflected by the room boundaries. With subs the first component is generally not the strongest unless you sit beside the sub.
Here's Brian's reply regarding setting different phase controls (such as sub1 at 0 degree and sub2 at 180 degree) for subs located equidistant:
Don't listen to those. Advise like that can work in a particular room and sub placement combination. It will cause cancellation of direct wave at your listening position. So what is left is the room reverberation (or in other words, echos in your room). The sound stage can completely collapse. That is not what we want.
You can try with your front speakers, if you connect them in phase, the sound stage is between your speakers. If you connect them out of phase, the sound stage becomes blurry. If your front speakers have excellent phase response, you will again hear very clear sound stage, but this time, the sound stage appears to be on your left and right sides with very little between the speakers. Now the sub is out of phase, but the front speakers are not, so what do we get? It is something in between. I for one think correct phase reponse is above everything. I have heard people wiring woofers out of phase with tweeters in order to fix an out of phase problem. This even happened to well known speaker designers. But those "fixes" have never become standard practices. In another words, you will never hear some speakers become outstanding because they wire the tweeter out of phase with woofers. They are....merely fixes.
Here's forumer LarryU's comment:
If the subs are equal distances from the listening position both subs need the phase set to Zero.
Considering this diagram as an example of a non-symmetry room:
... sub1 is next to a large window but sub2 is next to a brick wall. Both phase controls at zero? ...or may be different?