hbach

It could be the attenuator. Put it a bit separate and do a sniff test.

If you have a 100W attenuator with a Plexi at 8 you are probably hitting it with 120-150W. So in two hours of operating over the nominal maximum load the covering of the resistance will start to smell due to overheating. It's a sharp chemical burn smell like a soldering gun.
You mentioned eviction, so even if your attenuator is "big" enough, if you play at apartment levels with a cranked Plexi the attenuator is dissipating damn lots of energy. You are basically converting 150W into heat and 0.1W into sound. Heck even your speaker turns 90% of the energy into heat!
If you do the same with the 50Watters the attenuator only works half so no problem there.