Update (yet again): the Morpheus has been doing its thing in my system for about 12 days now, and an interesting thing has happened. Either it has opened up some after quite a few hours of run-time, or my ears and sensibilities have accustomed themselves to the way it presents the sound, or possibly a combination of both. Since it wasn't new when I bought it, I assumed it had been entirely burned in, but maybe not entirely. In any case, I'm really starting to love what I'm hearing now. The slight flatness I was hearing before has resolved itself into smoothness, and I don't find myself pining for the slightly rounder-but-less-precise sound of the AMR DAC that the Morpheus replaced. Had to do a bit of experimentation with inputs, as previously I had been running both the TV sound and my Cambridge Audio disc spinner through a Wyred4Sound Remedy Reclocker which converts everything to 24/96 resolution. It's a big help with TV sound which can only be connected through optical cable, but I found that the disc spinner sounded far more natural and appealing connected directly to the DAC via coax (I'd love to try AES/EBU, but the CA spinner doesn't offer that connection) and playing CDs at their native 16/44.1 resolution.
So I think the Sonnet Morpheus is going to keep its place in my system for a good long while. Falling in love with it was a slow process, but I'm there now. If I get itchy for further upgrade, I have a pretty good sense of which direction to go, but that won't be soon.
Thanks again to all for the helpful comments and information.