Now that I've put some time behind these, and got a comfortable listening volume dialed in, I'm ready to make some more in-depth impressions.
First off, mids aren't nearly as nice as the CD900STs. Not to say I'm disappointed, because I expected they wouldn't be as good, but it's still something worth mentioning. They don't have the ridiculous micro detail of the Sonys, and don't have the fullness either. In their own right they're still great in those regards, however. On some material they can be sibilant, but not everything (again, now that I got the volume dialed in)
With the CD900STs, I was so used to them at any note of a song I knew what I should set my amp to, and with these I had to dink around with it for a while. I doesn't really help that the bass is very recessed on these, so I can't really use drums or bass guitars are a volume reference point. I really didn't think a headphone could have less bass than the CD900STs, but these thoroughly proved me wrong. Drums and bass guitars have a very distant sound to them, so sometimes it can be a little hard to follow a bassline, and drums just lack any real thump, but it's surprisingly all very detailed.
Now, I know it sounds like I'm disappointed by these, or that I don't like them, but I really do. They just have some major differences that are going to take getting used to, and really, that's what I was after. I was so used to the way the CD900STs presented my music that it was all getting very dull for me. So what good things do these do? Well, plenty!
First of all, the soundstage is incredible. Very wide, but very deep as well, and offer a good deal of rear-projection. Much better, to me, than my old K601s in this regard. I was never able to get very good soundstage out of the K601s on my amp, and I think it might have had something to do with (a) the headphones having a fairly mellow high end to begin with and (b) they have an impedance spike in the highs, making my amp unable to provide sufficient power (it's a Little Dot I+, so it's just not built for high impedance headphones). My amp seems to play much nicer with the K501s, because they don't seen to have that impedance spike. Or at least if they do, the added treble can compensate for that.
And speaking of treble, it's just absolutely lovely. Very sparkly, but very rarely ever becomes harsh. Instruments like cymbals, high hats, acoustic guitars, and brass instruments have a very realistic sound to them, and it actually has me listening to more classical now. The CD900STs were just too dull to really enjoy classical on.
Oh, these also work very well with Dolby Headphone. I was listening to a few songs with it, and it's really neat, and has a more natural sound than my previous headphones had with it. Not that I'm going to be using it all the time, since it's still not perfect, but it's a neat little novelty.
I'll have to see how well these do games. I suspect very well!