The best HU for the price right now imo is the Clarion DXZ-835MP at www.thezeb.com ..I picked one up for my girlfriend's car and it is excellent. Great SQ, lots of features, and it has enough color options to match the interior. The retail was once $499, but you can get it authorized with a warranty for $220 shipped there. It's worth every penny.

 

I couldn't tell you how much the quality varies between LOCs since I haven't done A/B comparisons with them, but given that some people have reported that changing out the HU has made their SQ "1000x" better, I speculate that it may be due to a poor quality LOC, because at least with my system, there just isn't *that* much of a difference. The only other thing I can think of is that they weren't using an EQ and maybe having an alternative HU gives them a different frequency response curve that they're happier with. Or maybe I'm just hearing impaired after all the years of loud music.

I use the Peripheral Ven4 LOC, which seems to be well regarded by some of the posts I've seen about it on usenet and other car audio forums. No whine, no noise, just clean, strong output.