Can't comment on doom and sludge coz I don't play it, but I love my Rockerverb 50. Plenty of gain when the last gain stage kicks in, great crunch, and (this may be of no interest to you) RV's work brilliantly with single coils for clean / light overdrive.
The RV can't compete with Sue's Diezel VH4 for sheer gain, and the VH4 sounds much more contemporary when played side by side. But the RV has character. Of course the problem with character is it may not be one you like. :?
If you can get a straight swap I think it would be worth it for the experience if nothing else, the RV is a fun amp.
PS. Please don't hate me if you don't like it!