I personally find the SQ from the evo to be petty disappointing. I've used various headphones (Sennheiser, Grado, Shure (iem)) and none of them sound very good. I briefly tried mixzing's EQ and it didn't do much for me either, but that's probably because I've been using Cowon players for the past few years and I've become pretty spoiled (the sound on those things is really amazing).
(As an aside, this is actually one area that tempted me to jump ship for a galaxy s phone, as I've heard that their SQ is pretty amazing (Wolfson dac combined with decent Samsung sound processing), but, sadly, the epic 4g for whatever reason doesn't come with Samsung's improved music player/sound enhancements for the time being.)