xxx is a great sounding amp but i wouldnt swap my 5150 for anything.
recorded with both and xxx is a bit tighter and clearer sounding but 5150 has the thick crushing tone we all love it for.
If u find the 5150 is getting muddy beyond 4 (although that is v v loud!) then eq can be tweaked to fix that, turn resonance down a bit etc.
And those active eq's on xxx r a bit of a pain in the arse, tweaking has to be really slight and carefull, 5150 is pretty much fool proof! which i like.