tim -
Glad to hear you like the 501's. I had them for a while and agree that the imaging and soundstage are great. So much so that the crossfeed circuit on my Headroom amp did not have an appreciable effect on them! And those mids are so accurate I occasionally found myself turning my head to see the singer to my left or right. Very comfortable to wear and non-fatiguing to listen. But...
After a while I found myself wishing for that extra octave or two in the lower range. I knew it was in the music but the 501's just seemed to say "I can't go that low and do it well, so I just won't go there", and they stop. I soon found that this desire became a fatiguing factor in my listening of alot of music, if that makes any sense. Not all music necessarily. There is some music, particularly genres that highlight female vocals, with which the 501's just shine. But the rest, I was just wishing to have the mid-to-upper's of the 501's with just a bit more low-end "oomph" to round it out.
I was looking for a single-headphone solution, so I sold my AKG's and got a pair of Senn 580's (at almost twice the money). Now the Senn's are not as accurate in the midrange but I find that I just listen to the music and forget about the rest with these 580's. They are just a bit more "fun" for me. If I was able to keep multiple pairs of cans around and had the desire to switch phones depending on what music I listened to, then I would wish to still have the 501's. But I don't do that, so...
Maybe in the future I'll try one of the after-market cables or something.
Again, glad to hear you like'em though. Don't listen to me or anyone else for that matter. Do you plan to get an amp for them? If so what one(s)?