Please don’t give up on the Rockbox interface–it’s really pretty good, once you’re used to it.
And if by interface you mean the What’s Playing screen, the stock screen is pretty suckie–and it’s not what most people use. Instead, peruse the Themes page for your player type and see a whole new world out there! 
http://themes.rockbox.org/index.php?target=sansaclipzip
And by the way, you’re right in the Zip having been marketed as wav-compatible, without restriction.
