My experience with headphone amps is very limited and includes only those in my signature as well as those that I've auditioned in my home. The latter have been:
Burson 160; Woo 6SE; Luxman P-1; Graham Slee "Solo"; AE Nighthawk; Darkvoice 336 and 337. I was fortunate to also listen to the CSP-2 but not with my HD-800 and also with someone else's electronics. The Luxman and CSP-2 were very impressive, the others were also extremely satisfying but I decided to continue with the LF339 (slightly souped-up by my local electronic guru) which has given me the best sound for a reasonable price (except for the pricey tube parade I seem to engage in thanks to the folks over at the LF339 thread).