Rockbox shouldn't make DX50 sound different except for replacing bugs in Mango that alter SQ with hopefully no bugs (for example, early Sansa Clip firmwares have a bug that severely rolls off bass which Rockbox "fixes" by not replicating the bug). That said, it offers a fully functional parametric EQ mechanism so you can tune things to your liking if you want to go that way. There are quite a few other features that Rockbox offers like crossfeed, ReplayGain, working playlists, and so forth. It's much more than just a GUI change.
And fully functioning gapless, right? I don't think the 1.2.8 gapless has any issues but others have had different experiences...