we can't help with the gain. but the frequency can be were it sounds the best. Normally around 80 hz. Also make sure you have the low pass filter on. there is normally a switch labeled LP/HP/OFF you want it set to LP for subs. The gain you can play with, but you won't know what you are doing until you have a multi meter. Buy a cheap one that does A/C volts, (I think they all do.) and is digital. Following the instructions linked above will get you where you need to be.
Also did you wire the subs correctly? So the amp sees a load it can handle? If you have any questions about that let us know.