Fascinating problem.
To my amateur eye, the amount of clearance seems adequate.
If it was me, before ripping out sheetrock and doing more finish carpentry, I'd probably try to increase the ventilation by taking a hole-saw to the cabinet sides and possibly putting a couple quiet PC fans in there. Or you could even just try the fans at the back of the cabinet to see if MOVING the air in the existing clearance solves the problem.
Getting more clearance around the amps is going to be a PITA, and may or may not solve your problem if they're still "in a box".
