sas self-amped bazooka tube

I installed a sas bazooka base tube (8") amplified internally in the trunk of a mustang. Installed with 10 gauge power cable. I set the gain flawless match to the volume in car. The customer came back after 2 weeks and says the bass (amp) is cutting out on trips 2-3hrs. I adjusted the gain down a bit its nearly all the way down now. I couldn't get it to clip in the shop I tried for about 30 min. a 80% volume. He just called again and said it still happening. The Headunit has sub control I'm sure he cranks it as soon as he leaves.That shouldn't matter though when I tested it I had it up also. I have not had much experience with these, have not gotten any help from bazooka either. Any advice would be greatly appreciated. The trunk of the car rarely gets over 40 degrees. Winter.

Thanks

If you're using high level inputs, try using a line out converter with RCA's instead.

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My ex once told me I have a perfect face for radio.

Its hooked to sub output off the head unit. I set the head units cross. at 80hz(sounds best). Bazooka contacted me 2 days ago and said this product was not going to perform well being played at high volume for that long of a period time. I guess only 2 options left external amp, or upgrade to sub box and amp..