but, with a gound loop issue....between the two grounds (1) at the headunit, and (2) the amp- you can have a volt or two difference , evidently you misunderstood what I'm saying... I do know well enough about this...
This is why I run the extra wire... leave the headunit grounded to the headunit and connected (to the same connection) of the headunits gound and run this to the amps gound too- this will even out the difference.
Don't believe me.... been there, done that.
I never said don't use it (hu) or amp ground...Just add the wire to remove voltage difference.
the12volt.com has more for you to read....