I'm among the few XXX lovers here, but I expect you'd find it a tad more mushy in the lower register of an 8-string than a 6505. If you are wiling to give it a go and realize you just may end up trading heads later, you're coming out way ahead on this deal. Combos are impossible to move vs. heads and cabs. You'll be getting a bigger rig made of two more salable components. 5150 and XXX 4x12s are fucking monsters too.
Frankly, this borders on "too good to be true," but XXX heads can be hard to move for more than like $500 and those cabs don't go for much either, but assuming its all in good condition you're just lucky.