who made your box, is it prefab? i had the same problem my h/u clips at 33 out of 35, when i would be listening to music i would turn it to 18 and sounded good if i went up to 21 sounded nasty so even if i was able to go to 33 without clipping i had bad sound, the fix was building a new box and get rid of my prefab box, now i listen at 18 and the more i turn up my h/u the louder it gets and very clean sound too