does your kicker zx 750 have a bass knob? anyhow to answer your question i would say it is because of 1 or 2 problems, 1 yes your gain is to high i keep my gains at the lowest amount possible. or you have your sub wired wrong that 750 is perfect for a 12" L 7. if you do have the remote bass knob keep your gain down, and slightly use that bass remote until you can identify clipping of the sub. if your box is to small or big it can cause damage to the sub also this i found out the hard way. that smell is the voice coils heating up and getting ready to go. i never heard the boost had to be lower than the gain