I assume you're talking about an active high pass filter. Adjusting it lower and to a higher Q not only extended the low end response, but boosted it at the HP filter's corner.

My sealed subs are flat (in room) to 10 Hz. (also as low as I have tones to chart), but they can't take too much power at that point. It's too cool to feed a 10 Hz. tone into them and watch the cones move at 10 cycles per second while you only feel it and hear nothing.

I KNEW those dual 18 beasts could be flat to 10 Hz., corner loaded, in room!

Try DareDevil. I wasn't too fond of the flick, but in the first chapter at about 4:40 there is about 5 seconds of sub-sonic tone that actually moved my floor like the ocean.

Yeah...many times I've pictured the maniac Soundhound with 4-18s. It's a beautiful thing.