I can understand a bit of dimming, but this is just abnormal.

I'm not quite sure I understand what you mean by "grab the signal for the RE-Q right at the sub"

 

Tap the speaker wires at the factory sub, or woofer, or whatever GM put in there for low end. I am assuming that the T-harness you used went inline after the deck. Like Kevin said, the signal there will be low level and likely balanced so it's no good.

 

There is no factory subwoofer in the system. It consists of two 6x9's in the rear deck, components in the doors, and a center channel.

The t-harness utilizes the front midbass speaker as the wires used.

 

ok have you checked if the actual signal you are using as input has full frequency range? it may be crossed over and as a result you are being forced to crank the gains to reproduce the little bass that is left in the system

it may just be that the 6x9's are acting as subwoofers... can you hear highs coming from them? or just bass?

gm and a few other manufacturers are funny like that when factory amps are concerned

 

both mid bass speakers in the doors and rear deck 6x9's play highs, or should i say, 'mid' frequencies.

if you play a 25hz track, the mids and 6x9's go nuts with excursion, so they are not crossed over

 

I dunno... I'd suspect that the midbass speakers have some sort of contouring on them. Regardless of that, is the harness between the deck and the amp? If so, try grabbing the signal for the ReQ right off the 6x9s. Just do it quick to see if it solves the problem.

 

this is what I was getting at...

 

here is a link:
Subthump 5th Gen Add an Amp T-Harness for 2010 Camaro

we used it so that we didn't splice into the factory wiring and void any warranty issues. next time i see the guy, i will try just going into the rear speakers and see if that changes anything

 

ok just had a thought... try to use this but without the shield on the rca's.. I know sounds crazy.. but try it... feed the amp using only the center conductor of the rca's
had that problem on some cars like vw, bmw and bens with factory amp

 

so... i gotta cut apart a pair of nice RCA's?! LOL