I still find that Kempfs 1951-1956 Beethoven Sonatas sound good in mono with my Ultrasone HFI-780s because of the S Logic Plus effect (even Schnabel's 1935 sonatas are OK, although better with a little EQ to reduce the hiss). Know that Ultrasones do tend to color the sound a little (which might be a considered an understatement ).
I've never tried the spatialization DSP effects obo mentions (me thinks I need to with my other phones), but that sounds like a great help to improve the experience.
S-LOGIC is neat with a lot of things. But it isn't "tonally accurate" imho (the RS-1i, however, is). Ultrasone v Grado is a never-ending debate in my mind - both are lots of fun, one is more "enhanced" and one is more "natural" though. Sort of like listening to a good Pro Logic system versus a good stereo system.
A few years ago Ultrasone was starting to get pidgeonholed into "best for piano" - no idea what happened to that. I think every one I've heard does a good job, but I don't even want to think about running my PRO2900 + mono track. It probably works a lot better than I'm fearing though (I'm just imagining a super-railed image straight to the face). I just know that with conventional headphones (TBSE, RS-1, etc) a mono track will produce an extremely centered "brain cloud" effect, unless the set you've got has massive channel imbalance or one side is out of phase. It's detailed and accurate and all, but I don't like the experience.
Thanks to everyone who has replied so far. I am now seriously considering the Grado SR325is (although it will stretch my budget a bit). A very closely related and similarly priced headphone is the Alessandro-Grado Music Series TWO, so I'm considering that as well. It apparently has a better soundstage and less-harsh treble.
I would get the SR-225, or simply based on their reputation, the MS-2. I really didn't like the SR-325. Or the TBSE (because they're, you know, $50).