First off, that instruction that you followed is how to get maximum power out of your amp. It doesn't have anything to do with matching levels with the rest of your system.

I would take a pencil, mark where your process indicated max power was acheived, and then turn it down from there until you get where it sounds best.

Also, where is the bass EQ boost set? Try turning it off all the way (if that amp has it).

Make sure your HU bass setting is in the center.

Your Crossover is set to LP, 90-70 Hz or so, right?

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