Just to close the loop on this, my MC302 arrived this week. The MC302 is the better amp in just about every way (and it should be as it's 3x as expensive as the ST-500). Above all though, I was right in detecting a certain cold and detached quality to the ST-500. Kind of like the kid who gets the instrument technically right but isn't conveying emotion. The MC302 has much better mids, deeper, more satisfying bass, and I get way more emotionally "into" the music than I did when listening to the ST-500. It may lose to the ST-500 on the width of its soundstage, but I've only had it running a few hours.
Does make me wonder whether there's something to the oldschool criticism of Class D amps..