I'm the last person to talk amps with any authority, but I think the deep end of the pool presses an amp much harder than the 200hz and over levels. So, given your experience with amps, perhaps the issue is more an issue with your amp not having the headroom to dump a hard 50hz signal, whereas the sub has its own amp made to run in that end of the pool.

Just an uninformed guess. That's what you were looking for...right?

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire