I have found that sometimes the sound will be less boxy when it sits in a mix. I had this happen w/a Carvin V16, which has a very small cab. Once the bass and drums were going, much of that disappeared in terms of what the ear picked up. I don't know why that is, and I only am speaking from my experience w/that particular amp. I was playing blues rock, and that may have something to do w/my results (less low end thumping).
Otherwise, I'd say new cab.