Ryder, I have a two channel system with one sub. I agree with Byron that it is important to blend the sub with the mains with respect to frequency cut-off, phase and loudness. My sub is a Paradigm Signature Servo, which is self powered and has separate controls for frequency cut-off, phase and loudness.
I don't know how to optimize time delay (phase) as close as Byron (within one miilisecond) other than just by using my ears to minimize phase cancellation at the cross over point. As regards FR, my mains start to roll-off at 40-50Hz, so I set my sub cut-off frequency at 34-40Hz. So . . . after phase response/time delay and frequency cut-off is adjusted as best as I can, the only adjustment remaining is loudness. I generally don't fool with loudness very often, but may based on source material.
FWIW, using one sub in a two channel system presents two issues in my rig. The first issue relates to summing the stereo signal from the pre into mono for the sub. That topic has generated a lot of discussion is several threads that I participated in, e.g., impedance matching and asymetrical loading of the pre's balanced and SE outputs.
The other issue relates to the placement of the sub. I don't have a lot of placement options in my basement. However, I am able to place the sub in line with the fronts of the Mains and I aim the sub at my left ear. As a result, it's not to difficult to adjust phase. Even though it's hard to hear the directionality and stereo effect of very low frequency sound, I can actually feel (though not hear) high SPL in my left ear. A cheap SPL meter I bought from Radio Shack confirms this. There is another thread on A'gon which discusses the concerns of listening to music at sustained high SPLs.
Anyway that's my experience with a single sub in a two channel system. Hope it helps. Good luck.
