Thanks for the advice.
I noticed that the amps on PE don't mention they have a rumble filter, but many of them had the instructions for modifying the filiter so I inquired. Turns out that all of their HPSA series amps have the rumble filter. That includes their one rack amp and two plate amps. I have a customer who wants the rack amp for his sono-sub, so I'll give it a test run and let you all know if it works (and if it does I'll pick one up for myself).
Funny that infrasonic filters used to be big in sub marketing and now even the amps that have it don't advertise it
