ur box peaks at 44hz. it will play lower than that, but id be surprised if there was noticeable output below 35 or so. Also, it should play up to 80-90 fine before it fades off. I like my lpf at 85 personally (kenwood deck). alsoo, it seems as if you're complicating your settings. i would leave the bass setting on the HU flat, or at 0. it shouldnt do any damage to your door speakers as long as you dont go past 0, since it would then be boosting that signal. also, try to keep the SW level at 0 or below, as it too will boost signal and could lead to distortion. as to your gains, 3/4 should be fine. those amps are pretty good considering age. and honestly, if ANY amp sends dirty power with gains at 3/4 its beyond a terd. i say put the bass at 0 on the HU, and put gains at 3/4 on amp. also, keep bass boost off on the amp. I do however think a lower tuned box will help meet your goals.