Integrating subs into 2-channel and home theatre

I would like to upgrade my subwoofer to something more musical such as a B&W 750 (it's one of the few I have actually listened to) or something recommended. I have a fairly large room and in one section of my cabinet I am using an NAD M-15/M-25 pre-amp amplifier combination for home theatre. I am also still enjoying my traditional two-channel Threshold fet ten/hl / S-500 pre-amp amplifier combination.

Current main speakers are Mirage M-1's which do not suffer from lack of bass extension, however, I want to keep the options open for using a good musical sub, or stereo subs, with future speakers.

What would be the most accurate connection to use and could I keep the sub(s) connected, in some way, to both systems? Would the 750 support these connections? I do not mind disconnecting from home theatre to connect to the two-channel side if the case may be.

Or should I forget the fet ten, (purists don't attack me) run everything through the M-15 pre-amp, and maintain the S-500 for the front mains? I am open minded and not particularly on a strict budget.