how big is the box you said on the low notes it gets quieter? i just skeemed over your post. if thats the case there's several things that could be wrong id be more than willing to help you out just pm me if u cant seem to figure it out.
http://www.youtube.com/watch?v=aG-KU_LYypM
how to set your gain the technical propper way, i didnt belive how big of a differance this would make until i did it myself.
i would recommend getting this done asap, if you havent already.
1300 watts split into 2 channels but its a 1 channel amplifer for 2600 watts?
There are no split channels in a Monoblock.
I am not 100% sure you know what you are doing.
Its a monoblock with 2 output terminals so there is no 1300 watts x 2 making it 2600 watts into 1 channel. I am pretty sure all Brutus amplifiers are monoblocks.
So you are getting what ever your amplifiers RMS raiting out of both those channels, Alot of monoblocks have an extra set of terminals to keep the install looking clean.