Hi scottilad,
A few more answers to your many "burning" questions:
No, a lower-power or better-quality amp will not provide more accurate reproduction at low volume. You are experiencing a trait common to all human hearing--our sensitivity to low bass frequencies dramatically decreases at low listening levels.
The Fletcher-Munson effects referred to in a later post describe these effects, named after the researchers who investigated them. That's why Yamaha and some other early electronics manufactures included the so-called "loudness" control: as you decrease the volume, the loudness control gradually boosts low bass to compensate for our hearing losses (of bass) at low volume. It more or less works, but you can compensate by simply using the bass control on your receiver. Just boost the bass a bit (or reduce it) at low listening levels. "Loudness" buttons engage a fixed overly-generous bass boost and some treble boost. They're not very useful. A continuously variable separate "loudness" control can be quite useful.
Paradoxically, the smaller the speaker enclosure, the less efficient the speaker is and the more power it requires.
NAD has used "loose" power supplies for years. They are able to deliver up to 6 dB greater output for a second or two (or less) to handle sudden bursts of music without distortion, but can't deliver sustained continuous power. It suits the nature of a lot of music (but not all, especially compressed pop or rock music). Nor are NAD amps rolled off.
Apart from rare boneheaded engineering choices, most electronics these days are remarkably similar in performance. It's the last place I'd look for significant differences, although I've never reviewed an Optimus amp. Most of these amps are OEM'd by the same manufacturers in Taiwan, Japan or China, and when other factors are kept the same, they'll perform in similar fashion.
As long as stereo separation is maintained in the amp (and it always is these days) an amplifier has no effect on imaging. It's all in the speakers, no matter what you read in some of the "tweak" magazines.
Tube amps are a different breed. Subject to wild variations in frequency response, generous distortion, and interactions with the impedance curves of speakers, a tube amp is quite unpredictable. They can sound perfectly fine (and I've heard Jolida amps sound very pleasant), but performance begins to deteriorate the moment they're turned on, as the tubes age and their characteristics change. If you want to put up with that, fine, but an amp should simply be a "straight wire with gain" and not intrude or color the audio signal.
If you want greater midrange clarity, consider moving to a pair of M60ti's or M22ti's, the latter used with a subwoofer.
duff: I find listening from about 7 or 8 feet away is excellent in my room with the M22ti's. Space them apart about the same distance or a little less.
Regards,
