2006 gmc yukon w/ bose, lights dimming

I have a new Yukon with 5K miles on it.  I have the bose system, without the navigation or Denali package.  I tapped into the wiring harness near the amp for the hi/low converter...I have a pretty weak systm, i just wanted to be able to hear more deep bass over the stock system.  Anyways, I added a 200 Watt RMS amp to 2 svc RF P-1 12's max 300 watt rms.  I used 8ga wiring and when the gain is all the way up, my interior lights dim really bad, i don't really notice it in the headlights...So the only way I think the system sounds good, is when the gain is all the way up...ANY SUGESTIONS WOULD BE GREATLY APPRECIATED!!!  My truck is brand new, I thought it would handle this small system!!

THANKS!!!!!!!!!!!!

Thanks for the quick reply!

My system is pretty generic, I bought it at Best Buy.  The box is a pre-fab'd Q-Logic box.  I have it right behind the third row, firing directly at the tailgate of my SUV.  I  don't have the XL so space is limited as far as the box.  I really like the way average 12's sound vs: average 10's...I know it's tight, i want to build a custom box soon. 

So should I just turn the gain down, so the lights don't dim?  I had a very similar system in my 99 Tahoe, i had some older rally series boston acqustic 12's....my lights never dim'd and the gain was all the way up on that??

Thanks again for your help!!!  I am trying to learn about these things and this forum is very helpful!!