I gig my NX2 212T often with either my Streamliner 900 or Shuttle 9.2 w/Am St P5 w/nylon tapes. I always enjoy my tone with either. I don't get PA support in a TX blues band and I've used this setup at outdoor gigs. Never had an issue with this rig but I've never pushed it into speaker breakup or distort, even when I dig in hard. And I feel that band can get loud, what with guitard and keytard, pushing their lows some. But I know volume is relative/subjective. THough I feel I've cranked this rig pretty good and its' always sounded pretty great. Maybe OP should open cab up and sniff the inside of cab and look inside as well to see if he spots any burnt wiring, wires hanging against drivers, etc.
Come back and let us know if you found anything.