Gain issues

Setting gain on sub amp. Everything flat off , Hu amp same etc . Hu 2/3 volume running a 50 hz test tone gain turned to minimum . Using Bluetooth for test tone .
Problem as soon as I turn on the test tone my subs it sounds like to me are playing. The tone really loud before I set gain. Really can't even turn Hu up to 2/3 it's too loud . Getting 13.4 from battery 13 .2 from remote wire. Can not touch gain at all . When I played music with minimal gain the bass sounds like it's coming from the car behind me , not up front under me so to speak . Triple checked my grounds, RCA's, different Subs .