Good grief, I thought I'd replied to this thread two months ago. The ingratitude! Life has disrupted my listening and internet time repeatedly and I let this slip through the cracks.

As far as I can tell, the digital volume on the Oppo was the culprit; and I'm embarrassed it didn't occur to me sooner. Actually, I think I _had_ toggled between 'fixed' and 'variable' volume, but the experiment wasn't controlled enough to yield meaningful results. I've very lately been throwing dozens of very different kinds of recordings at the Sierra-2's and the results have been so much more beautiful than I remember from even two months ago. It might have been a weird, peristent, 'placebic' buyer's remorse as well. I rarely pay this much for anything at once. Whatever the case, I feel a lot better now than before. I'd written off the notion of speakers needing "burning in", but my feelings about the speakers are much better now.

Thanks to everyone for the suggestions. Changing speaker placement has been almost as easy, and I think it's yielded some significant results.

I might also soon be in the market for a more powerful amp. For a variety of reasons, that might be an AVR rather than another integrated or separates....but the debate over the (dis)advantages of each of these leaves me dizzy and further confused, so I need to keep researching. I will probably revive a receiver recommendation thread to ask about relatively inexpensive options.
Apropos sludgeogre's comment above, I don't use a Class D, just a regular A/B 40w integrated amp. I am still concerned about the possibility of clipping, though I've been using the NIOSH sound meter app (albeit on an Iphone 5's internal mic, no external mic....please don't scoff!) and brief highest peaks almost never exceed ~85db(A), with ~80% of listening remaining under ~75db(A). Unless this method of measurement is worthless, that's where I stand with volume. An SPL meter will have to go on the acquisition list.

Sorry for the delay, but thanks everyone!