Played it with my XCan 3 (modded but no PSU) for 5 hours and my impressions are as follows playing Bjork Greatest Hits.
1. Harsh treble and highs
2. Very sibilant
3. Bass is quite audible, not prominent but sounds tight/more texture while 595's bass has more weight. I could follow the bass line easily without any effort. I would consider the bass to be well balanced.
4. Very wide soundstage
5. More difficult to drive then HD 595. I need to turn the volume pot less then halfway to get decent sound. However, I think it is driven well as there is no background noise (the type of noise when you turn the volume knob way high). I demoed it on a ROTEL intergrated amp and just 1/4 quarter turn of the knob produced alot of background noise.
6. Not as confortable as HD 595 but still good.
I'm gonna leave it for 2 weeks 24/7 playing and will listen again.
I got my K501 several days ago. My initial impression was similar to yours: very harsh sound, very little bass. After about 30 hours I can tell the headphones are becoming more smooth and the bass also has improved slightly. I expect K501 sound to "stabilise" after another week or two (based on the experience with K271).
I'm using K501 (as any other headphones I own) with homemade class A amplifier (class A is The King!). The headphones do not sound particularly involving on rock but I've been recently re-discovering! my classical music collection with them. The piano and violins sound simply amazing!
The other day I tried to compare K501 with HD600, RS1 and K271 on _classical_ music. K271 are simply not suitable for the task (although they are great for rock, not bad for electronica, and not too shabby for jazz). RS1 are _great_ for acoustic instruments as long as there is not too many instruments involved. But they sound definitely warmer and more colored than K501. After listening to Chopin, Vivaldi and Bach's piano works using K501, switching back to HD600 sounded, basically, disappointing (I never thought I would say that one day!).
In K501 I discovered "perfect" headphones for classical music (well, at least among < $500 cans).
But, to be honest, it is not the _best_ headphone ever. I don't think the _best_ headphone exists...
So far, my (preference) picks for the best (perfect?) synergy between music and headphones are:
1. John Coltrane and Jonny Hartman -> Sennheiser HD600 (!)
2. Kate Bush -> Sennheiser HD600
3. Diana Krall (few different CDs) -> Grado RS1 (!)
4. Beethowen 9 Symphonies (Karajan/60-ties recording) -> Grado RS1
5. Beethowen 9 Symphonies (Gardiner) -> Sennheiser HD600
6. Phil Collins "The Best" -> Grado RS1
7. piano works (Chopin, Bach, etc.) -> AKG K501 (!)
8. strings (violins; Vivaldi) -> AKG K501 (!)
If memory serves I recall some older posts that regarded the Xcans as being inherently bright, and not a very good match for the 501. My pair of 501's was a little sibilant on the top end when brand new, but they smoothed out nicely over two weeks of regular use(including playing pink noise and a wide assortment of music through them while I slept).
The original XCan 3 is bright, but my modded XCan is suppose to be transparent. I said 'suppose to be' as I've had not finish burning the newly installed caps and tube (176 hours out of target of 500 hours). Anyway it sounds transparent now and the harshness is gone when used with my other headphones.
To be fair, I expect the caps to fluctuate in sound quality (Blackgates are suppose to be notorious in breaking in) and I will only listen critically to the AKG after the whole break in process is complete.