I've been using Rockbox on the DX50 for a while now and really love it, i agree it sounds better than the stock and Sound unlocked firmwares.
In the sound setting menu i've tweaked the Bass +8, Treble +12, Channel Config Custom with Stereo Width @ 145 % - I love the sound! Not yet touched the Parametric EQ at all as it looks pretty complicated but i think i might have a play with it soon!
My Favorite Theme is Redux - i think it looks awesome on the DX50
Also not sure if you've figured it out yet but after applying the theme turn on the 3x3 touch grid for your own sanity (General Settings >> Display >> Touch Screen Settings >> Touchscreen Mode). I find the normal touchscreen controls in Rockbox are a nightmare!
After setting 3x3 grid mode imagine the screen split in 3 rows of 3 squares, here's how it works :) Enjoy!