the HU is the very first thing in the order of things in the audio system. if it sends crap on to the amps, they will reproduce crap and send it to the speakers, and you will hear crap.
i noticed a huge difference when i switched from the Panasonic CQ-VD7001U to the Alpine CDA-7998. it sounds so much better!! but the screen is messed up in the alpine and i'm trying to get with the seller to resolve the issue. so i don't know where the EQ's and Levels are and can't change them since i can't see them, lol, but if it sounds this good now, i can't wait to get a new unit and do some tuneing!!