Probably with EQ. Terms for tone is totally subjective. I have a cabinet that I would term as boxy. I finally have it figured out. I had to roll off upper mids (700-900 mHz) to get it to sound right for me. A para-eq can be a godsend for tone problems.
I agree, I would experiment with EQ before trying any other types of effect or processor to correct what you're hearing. Especially reverb, which IMO would just take your boxy sound and smear it around more.
Alright, I'll try some EQ experimentation. I have a Boss GEB-7 that I haven't had in my signal-chain for a while.
Thanks for the replies!
that's a good start, if you don't quite get the result you're looking for, see if you can get your hands on a 31 band rack eq to get really good control of the bad frequencies. once you figure that out you can move on to a parametric and cut the bad frequencies with razor like precision!