True, that's what the damping factors of these amps are, but that's not likely the reason the Sphinx is sounding better to your ears, because one of the choicest amplifier types for Omegas is a SET which traditionally have very low damping factor. All Omega drivers are very light and don't really benefit from high damping factor. As far as the M3i not working well with Omegas, it's likely nothing more than the M3i's sonic signature and not an impedance issue or some electrical mismatch as Omegas present a very benign 8 ohm load and are very easy to drive.
Glad the Sphinx is working out mrvco. Class D has come a long way. I personally love the sound of the Temple Audio Bantam Gold Class D amp. I would love to hear a Sphinx one day, as it looks like an incredible bargain, and made in the USA too.
I've said this elsewhere before : In my experience, class A/B amps tend to sound best when pushed a bit, say roughly 10% of their power rating constantly. So if you have a class A/B amp rated at 100 wpc, it will sound best when outputting something closer to a constant 10 wpc. Speakers that are harder to drive will certainly help the amp perform in a more ideal way / in its "sweet spot." This isn't a rule of thumb or anything, as obviously there are too many parameters to consider between amp and speaker pairing (considering the multitude of designs out there) to make a real blanket statement. Note : Roger Modjeski found similar results in some tests he conducted and offered that this can be part of the reason less expensive, lower output amps can sound better than their bigger brethren in a system, given proper consideration to speaker / amp / room size / listening distance / and typical listening levels. I whole heartedly agree with this. While it is easier to just go buy the biggest amp you can afford and be done, it doesn't mean you're necessarily going to get the best performance. It's possible, but going this route offers no guarantee, whereas I can pretty much guarantee when one carefully considers the parameters I've listed, the chances of getting it "right" are much better.
With all that said, something interesting is that I have found Class D amps and Class A amps to not suffer the same "necessity to be pushed a bit" nature of Class A/B to sound great, if not their best. They tend to provide the best aspects of their sonic character from the first fraction of a watt and beyond. So, different topologies seem to have their inherent advantages (for various speaker pairings) that isn't really obvious on paper. I suspect this has a bit to do with what you're finding you like about the Sphinx vs the M3i with your Omegas. With your Omegas, the M3i isn't being pushed into its sweet spot operating range and probably sounds thin, I'm guessing. It isn't a bad amp. It's just that it likely works more happily with less simple loads where it can push some woofers around a bit. Meanwhile the Sphinx is more than happy to show up to the party whether it's just background music in a casual lounge or a full-on dance club.