it is best to go pre-out to self-powered subs, then line level (less low bass) back to the preamp to amp. thus neither the amp or main speakers try to make low bass. that will help you with the carver 275 driving your 88 sensitivity mains which I imagine already sound real good. If you ever want to try a lower powered amp, you would be ready.
your existing preamp lacks pre-out, main in, so other changes ... perhaps think long range.
for home theater, small office, I use only 1 sub, my main music system has 15" woofers which you could think of as a stereo pair of subs. bass can definitely be directional
music system: I reccomend self-powered, front facing, no ports, stereo pair of matching subs. Subs located adjacent to the mains. It is both the fundamentals and the overtones of bass notes that give directionality.
you could use a Y splitter to go to two self-powered subs.