Alan, thanks for your carefully considered reply. I feel very privileged to have your assistance, and that of the others who've taken the time to assist me. And I am taking all advice into consideration, but not necessarily acting on them all (one cannot, since in all such things, some options are mutually exclusive).

That's interesting about the 'polarity reversal' topic. When NAD came out with the C390DD, I believe it was another manifestation of NAD's approach to bringing higher-end technology to customers on a stricter budget. So the C390DD is perceived as being a 'black box' offspring of the M2. So I suspect that the polarity reversal menu item trickled down from the M2. Another, to my mind, less that truly useful feature is that on the back of the C390 there are 2 pairs of speaker binding posts. But they are not for "Speaker A / B" selection. Instead they are for bi-wiring. I don't know what to make of this attention to $8,000 speaker cables and $400 power cords. But I do know that when I get my system to the point where these are the next steps in the pursuit, I intend to pack it in and start listening for the joy of it (not that I don't already!).

Although I really like my C390, I do have some issues with features that are non-essential to it's DAC/amplifier premise.

Regarding equalization, to which Ken C referred, I do not use any active components in EQ. I have used Room Equalization Wizard (REW) to calculate filter values. REW is not an active player in the signal path. It is something like Odyssey in that using a computer and mic attached to the amp, one plays sweeps through the system.

REW running on the computer then calculates filter values to compensate for room factors. My amp permits adjustments at 6 fixed frequencies, 40/60/90/120/180/240 hz. Once I've done that, I enter those values into my amp and put REW aside. The values REW calculated are in the range of +2 / -6 dB for my room, and when I toggle EQ in/out, the sound is better with EQ enabled. Playback does not involve REW.

Of course, the values REW calculates depend upon where the speakers are located in the room and where the microphone is positioned (i.e. the intended listening position) at the time the sweeps / calculations are performed.

It is true that each time I change something in the room, I should redo the REW tuning with EQ set off on my amp so that REW can calculate new values. I haven't been doing so for minor adjustments like moving the LP back or forward a few feet or removing toe-in.

I'm enjoying this conversation, so if it continues I will not be disappointed. On the other hand, I think my original question has been answered along these lines:

Yes, speakers can be too big for a room. But in my case the M80's should be quite workable in my room. In other words, I am not pursuing something that is impossible under my circumstances.

All of this makes me feel good in having chosen the M80's and the C390DD.

btw, IMHO the C390DD outfitted with the DD-BluOS plug-in module is a beautiful means of going all-digital. Controlled by the NAD-supplied tablet app, one has access to locally stored audio files up to 192/24 resolution, streaming "radio" stations and streaming on-demand sources including Spotify and Tidal.

Cheers everyone!
Tim