I am new to these home theater receivers, but I don't know how the
subwoofer is supposed to be powered. For example, the AVR-686 has pre
out terminals for a subwoofer. How is the subwoofer to be powered? A
separate amp? Don't any of the Denons have a power amp for the
subwoofer, or am I missing something here?

Second question. I have HORRIBLE acoustics in my great room with high
ceilings. Some of the Denon receivers have a microphone that they use
to automatically set the system up. With this setup and up to 7
speakers, should I be able to understand the words coming from my TV
that I can't seem to understand now even with my TV attached to a
stereo receiver driving a Bose speaker system AND its own speakers?