Well last night was kind of an eye opener. I swapped out the Model Sixes for the MC300s. I also swapped in the AVR-1907, leveled out the channels, in Direct mode. Turns out I didn't need a TV to do everything I needed to the AVR, but using the remote sure helped. You can only get things about 75% of the way by using the front panel alone. So that was a success in itself. Right now my iMac is connected via analog; it will be optical tomorrow, but I suspect that won't change things too too much.
As it is, these changes were substantial. We agree that the subwoofer is needed no matter which of my speakers I work with; the MC300s are a far better match to my sub than the Model Sixes. You could just tell they were happier together. It was kind of like lifting a light fog off the sound, going from Model Six to MC300. I've clearly turned in a good direction. I was sitting there last night really thinking about the advice given here regarding buying some different speakers. I can't help but wonder what kind of eye opener that would be. Right now, the amplification side of things has me reasonably satisfied; I am generally more of a lower volume listener, so I was yearning for a "loudness" button that I don't have to fill things out without cranking it, but I suspect someone here will tell me with a superior speaker, I'll not require such a feature.