I know a little bit about Rockbox, but I've never rockboxed a device. I'm definitely considering doing it to my DX50, but I'm curious about the change in sound signature. Can anyone do a comparison between stock 1.5 and rockbox on a flat EQ?
SQ comparison is too subjective. Not to mention, not everyone listen to the same music as you do and headphones you have. If you are really curious you should do your own comparison. It will be fun.
Rockbox's main objective is to extend the players function and versatility over the stock software, not to make it sound different. The default setting is flat which should be how the player is suppose to sound like. But of course you can do so with the EQ support rockbox has.