Sub completely cutting off at higher frequencies?

[Sage]

So here the deal. Today I got my neighbors weedeater fixed since it wouldn't run right and they gave me some stuff from their old truck. A JL audio 12 inch sub in enclosed box ( downfiring for a car with a trunk) 250 watt single channel amp, and a equalizer. My current set up was two visonik tens with a 500 watt amp single channel. ( it's a modified MA audio amp I beefed up) with the tens my truck bumped HARD. Mirrors shake, dash shakes, it's awsome. I switched the twelve in and I fell in love instantly, after playing around I got it tuned to where it shakes the Windows and stuff in the house which is 50 feet away. Here's my problem. The sub completely cuts out on the higher mid bass stuff. On the low deep stuff it does AWSOME. I'm talking the seats flex and my shirt flexes. Anything above a certain level it just drops off, or tries to make it but distorts. A buddy of mine thought this was weird since adjusting the low filter didn't help at all. It only produces the low stuff. I'm running it at 400 watts right now, someone suggested maybe I'm bottoming it out, but it sounds so good when it's low with zero distortion. Adjusting the frequency on the radio to 70hz is where it appears to have the best excursion but still cuts out at the higher stuff. My gains are at fifty percent, bass boost zero, and bass set to -7 on my alpine. It's okay, but I can't figure out why it only works on super low freqs.

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