WHAT THE HELL ? my amp dosnt have a gain control ?

thats what the amp you bought came with. a sub out on the H\U would be better, but a lpf and adjustable cutoff should work. input sensitivity is the gain. all amps have a preset internal gain that cannot be changed, when you adjust the gain, you are not adjusting the output level, you are adjusting the input level, more in, more out. so the the gain would more accurately be called the input sensitivity or input level.