I have played a number of late model (Rev E) RD 50s, and the Distortion mode is quite usable, especially with the right speaker installed. To get an earlier model (Rev A) model to sound as good, is it recommended to copy the limiter circuit from the Rev E schematic, or perform some of the other mods mentioned here and elsewhere on the net? Some seem quite invasive, don't want to head down a path I don't have to.
BTW, the speakers that really seem to work well with these amps are the WGS Reaper HP (12" high power handling clone of the Celestion G12H 70th Anniversary Special Edition) or the WGS G10C (their improved classic 10" Jensen voiced speaker). Celestion Alnico Golds (10" or 12") are also excellent, if more expensive choices, especially if you like a smoother attack and more blended, 3D sound.