Thanks for the suggestions JD (and 3.0charlie)! The NAD D3020 was almost exactly what I was looking for, but the crossover frequency- look ma, I learned a new term :haha: - is potentially a little high. Manual was somewhat vague.
I think I found the best compromise, a Yamaha integrated amplifier (A-S301). It's got an optical in and a sub-out, which attenuates signals over 90 Hz. Plus, if I understand correctly, it could be made to work with my computer and TV, if I ever want to upgrade my soundbar to a pair of passive speakers. Either I would need an optical switch, or just change the input as needed.
Only real down side that I can see so far-still over-thinking everything- is that it's huge.
After more time last night than I care to admit.... All the small desktop-style amps mentioned at overclock.net, etc that I had wanted to buy either lacked an optical in, had misleading specs or were of dubious quality/longevity.
what's the reasoning behind not using an AVR?