Hello mdrew,
The urge to turn up the M80s is natural because of a characteristic of human hearing. At low volume our ears are quite insensitive to deep bass frequencies, an effect scientifically documented in the Fletcher-Munson curves. Anyway, for some years many receivers and preamps included a "loudness" control, separate from the volume control, which compensated for this trait of human hearing. You would pre-set the volume to your usual loud level, then turn down the loudness control, which automatically increased bass boost the more you turned it down to compensate for our insensitivity to bass at low listening levels. Some Yamaha gear may still include a Loudness control. You can compensate easily by simply boosting the bass to your M80ti's on your H/K 7200 at low volume.
No, you don't need an extra amplifier to run a sub. Virtually all subs are "active" and contain their own built-in power amp. You would connect your Zone 2 speaker cables to the speaker level inputs on the sub, which extract the deep bass, then pass on the signals with a second set of cables to your M80ti's. But the same phenomenon still operates. At low volume, you will want to turn up the sub volume to compensate for your hearing of deep bass.
Yes, physics always dictates sub limits: the larger the box and the driver diameter (they go hand in hand), the deeper the bass frequencies and the greater the output. The larger the room, the bigger the sub you need. It's a pressure pump and does not care where you sit. It must pressurize the room volume it "sees", so larger rooms need a larger sub (or two).
This explanation overlooks many aspects of sub design, but the basics are covered.
Regards,
