If it happens with so many amps with/without the EQ in the chain, the simplest explanation is that there's something funky going on with the speakers. The amps could all have a problem, but for them all to exhibit the EXACT same symptom... highly unlikely.
It makes me wonder if the port tuning/box design on those Magnavox speakers is so bad that it's causing a low frequency standing wave inside the box or something that gets out of control at high volumes.