is it the input level?u need 2 turn all yer bass,treble setings 2 0,then turn yer stereo up 2 around 25-30(depending on what it goes up2)now,turn the screw untill it start 2 distort,when it does slow turn back untill it becomes clear again.u might have 2 have a little play but then when u turn the volume back down n stick some bass on there it should sound sweet.thats what i did anyhow n mine sounds awesome.....hope that helps
i try explain this is straight for and non technical as possible alot like the ohter post above.
pull out the fuses or what ever so ure amp and sub are not powered up if it already wired up
as the man said above turn the bass headunit setting to 0 and turn up ure volume on the headunit until it starts to distort ure speakers, once this happens turn it down a little till it become clear again thats ure max volume setting for ure headunit.
rember the volume number and turn off the engine/headunit
now wire up or put fuses back in the amp
now the 3 knob will be ure
1) gin knob - how loud it gonna be
2) low pass filter adjustment knob- what kind of bass u want ( also depends on size of sub)
3) sub sonic filter knob- tweaking ure sub sound
now make sure the gain knob is set to minium and ure low pass filter is turned on (be alittle switch) and set ure low pass filter adjustment knob to half way between min and max (which should be 50hz) and turn the sub sonic filter off.
4) turn on the headunit and set it to the max volume number and turn on the rear fader so u cant hear the speakers any more but the volume is still at max,(assuming u using the rear low level output) now turn up the gain slowly until ure subs start to distort (playing a bassy song)
5) when they start to distort turn the gain slightly down again as beofre until it becomes clear again,once agin thats ure max setting now play around with the low pass filter adjustment until u get the right sound ure after (i recomend 45- 50 hx for 12's) after that turn on the sub sonic filter andplay with that till ure happy.
6) finally turn down the headunit volume turn the fadar back to zero and crank it up and admire ure tuned subs
ps.. post kinda the same as above just a little more detailed hope it ok for u and that the guy above dosnt sue for copyright