Re: gain and frequency?
x2 on the gain setting tutorial. Please read it. When the gain is too high, the amp is clipping, causing your sub to sound awful. Very bad for the sub and amp.
Make sure you have the amp to low pass, not high pass. Should be a switch, right? Now set the frequency knob somewhere between 70 and 100. That's what I'd suggest. Do you have stock speakers? If you set the frequency too low, you will have a gap in your frequency response and it won't blend with your speakers. If you set it too high, the sub will produce frequencies that you don't want it to, and could sound funny. Play with it.
Running fully active with US Acoustics 4050 and 4060 Zed built amps
ID OEM 6.5" mids and Seas 27TAFNC/G aluminum tweeters
12" Dayton Titanic in 1.2 sealed powered by HS 2.125