Thanks everyone for writing back. I had both the 505u and the 550A II in my system and wanted to briefly post my results. First off, both are fantastic and I could easily live with both. As is always the case, it comes down to my own listening preferences (both in terms of type of music and kind of sound) combined with the rest of gear.
The 505u was very dynamic, with tuneful, deep bass and shimmering highs. It had more than enough power for my Devore 9's, and as with the 550A I feel like the wattage is very conservatively rated. Pace was particularly good, and the sound was very engaging. My little digs, which again are only in comparison to the 550A, are that the speed and dynamism of the 505 seems a little over-kill at times. Cymbals have a little too much attack, and in generla it feels like the leading edges are favored too much, making things almost sound too zippy. This was apparent in violins, which didn't sound as pure as with the 550A. This also makes the 505 sound a little hi-fi than the 550, which sounded more natural to my ear. It's almost like when you see really good computer animated versions of humans, and it's really cool but also a little creepy or not right. This is a REALLY minor version of that, and only by comparison, but in the end the 550 seemed the more human of the two.
The 550 was definitely preferable on the top end for me. It didn't have extra sizzle of the 505, but I found that a much more natural presentation. Furthermore, the bass, while not a zippy as the 505, sounded more like an actual instrument, than a recording of one. It didn't sound thick to me at all, just fuller, in a truly satisfying way. Vocals sounded hollower on the 505 than the 550, as did strings.
So, to make a long story short (and if anyone is interested in my notes, let me know and I can forward them on) I thought the 550 was a better choice in my system. If I had hard-to-drive speakers, I would without hesitation get a 505, or try the 507, but I'm lucky in that respect. Again, my digs at the 505 shouldn't be taken out of context, as it was a seriously good amp, but in the end the 550 was better by comparison.