|Albau: The Creek OBH-11 seems to be the lowest cost answer here and solid bass to boot, it may be a hard combo to beat. Have you tried any other amps with the 501?
nick danger (somebody else also asked this question), besides OBH-11 I've tried K501 and HD600 with MG Head DT and TA.
TA: I love it and use it at the office with MSII but, imo, it simply can't handle AKG and Senns properly, especially very hard to drive K501. With K501 TA simply didn't add much, except for darkening, compared to 15mW amp of my RS-3904 - same tinny, weak, unsatisfactory sound. My home Yamaha's headjack did much better. TA has less trouble driving HD600 but, imo, these 2 are really a bad match - rolling highs off already dark Senns makes for kinda dusty sound with bass disproportionally sticking out.
OBH11: As I said it's a match made in heaven with K501. It's fine with HD600 but compared with MG Head DT I felt soundstage shrinken and sound still too warm.
MG Head DT: It was on loan and, sorry, I don't remeber what tubes it had. Owner said tubes were very expensive and good. Anyway, I felt that HD600 opened up and blossomed with this amp compared to Creek. Everything became right - ballanced & smooth, with controolled bass, nice imaging. K501 were also fine but with Creek they had "sparkle" in mids that was missing with MG Head.
As a result among two couples made in heaven I've chosen the K501/Creek one. I saw that both combos are extraordinary but felt that AKG/Creek fit best with my personal preferences and music I like. Confort also played its role. K501 are the most comfortable (and good looking) headphones I saw.