Car Audio Amplifier ZE1000.1 2000W Monoblock Class D MOSFET Subwoofer Audio, 1-4 Ohm Stable, Low Pass Crossover, Mosfet Power Supply, Stereo

Fitted in a Jaguar Xf 2012, signal from factory rear speakers to line input of amp, driving Alpine Sub 12 in Bandpass Box, amp is being bridged for mono, not impressed had an Alpine 750watt before this, This amp is nowhere near half the power of the Alpine, I have to have the gain maxed, bass boost maxed, log is setup, subsonic filter off, and yes it does work, sounds like 100 Watt Amp. Most reviews and YouTube reviews say this is good, am I doing something wrong here. I'm 52, been a bass head since days of Tandy, I've had Alpines, Rockford's, phoenix Gold, even Sherwood, this Amp is just not that good. I'm going to change to a line to RCA and test again. If no improvement will return.