Oh yeah -- the BFD input level meter! Hmmm sounds like your AVR sub line level drive is OK then. I've read where the Behringer amps have a input that can handle home audio line levels (unlike other pro amps).
So the clipping is in the EP2500. IME I've just gotten occassional brief flickers of the red clipping LEDs on my Mackie for extreme bass scenes -- WOTW tripods, opening of Serenity, 1812 cannons, etc.
I'm driving a single 15" Tumult that is rated for 1000W at the voice coil with the Mackie bridged to output 1400W into 4 ohms. And I am able to bottom my sub when I run +10 dB over reference. So I set the Mackie's high pass rumble filter at ~18Hz and try to stay at reference level. Using it in a big 3000+ cubic ft basement room
Seems like your EP2500 driving 4 ohms is running out of oomfp before your 4 RL-P15's.
Did you think of/consider wiring your array to 8 ohms instead? Much easier for the amp to drive -- but half the drive current pulled from the amp.
Bob