oooh, I that should be nice! I though of doing that with my DT 880, the OPA2227 in the c421 [or the dual OPA2227] are very warm chips so they should compliment the already warm NOS DAC in the Hm-601. BE WARE THOUGH, I might not use a warm Amp with an already warm DAP UNLESS your headphones are them selvs TRANSPARENT or cold! The Hm-601 is REALLY good with Cold-> neutral cans! Anything that already has RICH mids might sound kinda bad <3
Still :D I want to get that one my self! I got an Indeed G3 with a 6992 EH and it sounded very nice! Simmilar to my cMoy but with a little better deail in the bass and a little more highs!
Still C421 should be a nice match,
and the Hm-601 is a GREAT dap, it had more Bass Response than my old iPod Nano 3d gen!
but DO NOT USE THE Hm601 EQ... it's VERY well VERY COLORED... BADLY COLORED. Stick with "no eq'' but optimally use the LIne out when using the Hm-601 with an amp!