They play perfectly coherent, just as if the main speakers have stepped up in size and are now capable of a truly full-range sound. As these subwoofers only handle the bass from the point where the main speakers are naturally rolling off in the bass, they only add bass extension, while the sound of your main speakers remains the same as before. I often hear people saying they don't like how the bass sounds when altered by low-pass filtering the main speakers, but that is never a problem when the subwoofers are only taking care of the lowest bass, which the main speakers can't handle on their own.
I have positioned each subwoofer directly outside its corresponding main speaker, ensuring they are physically playing in phase by making fine adjustments to the positioning of the subwoofers.
You could use REW measurements to adjust the placement, but the way REL suggests works fine, as it's quite easy to hear when the subwoofer plays perfectly in-phase with the corresponding main speaker, while listening to a music track with a repeated bass sound. You could use any track of your choice, but the one REL suggests works great, which is the track “Cosmo...Old Friend” from the movie soundtrack album of Sneakers. As already said, you will easily hear, while moving the subwoofer in small steps, when the subwoofer plays perfectly with the main speaker. The listening process is also quite fun, and at the same time, you get the chance to learn how it sounds when the subwoofer plays in perfect phase with the main speaker. Do this for one subwoofer/main speaker at a time, by turning off/disconnecting the left or the right channel.
It doesn't take much time to ruffly integrate these subwoofers seamlessly with the main speakers, and then you just have to fine-tune the crossover and volume level settings over the next couple of weeks, while listening to music. It's a good chance that you want some small differences in the settings for the left subwoofer compared to the settings of the right subwoofer.
After the initial setup period, you don't have to fiddle with the settings anymore, as you will find it sounds well-balanced and correct with everything you play, which seems to be another problem people often have when they don't get the subwoofers to integrate seamlessly with the main speakers. With subwoofers that are only taking care of the low bass extension, this is very easy to get right.
That shouldn't be a problem. Just connect the high-level connection cable from each subwoofer to each binding post on each amplifier, left for left and right for right, as you connect each of your main speakers. I use a class A/B dual mono integrated amplifier in my system, and the subwoofers are connected in a stereo configuration where the left and right channels are kept fully discreet. The high-level cable that comes with the subwoofers has three cables: one red and one yellow cable that go to the plus (+) binding post on the amplifier, and one black cable that goes to the minus (-) binding post.
As the amplifiers in the REL S/510 and the S/810 are both class D, and your amplifiers are class A/B, as mine, you will likely need to switch the phase switch on the subwoofers to 180 degrees. Class D amplifiers usually invert the signal compared to class A/B amps.
