I've never been a real technical guy but when I worked installing car audio the boss always made a big point of making us keep the grounds as one of the biggest parts of any install. The 18" rule was to keep the least resistance running through the wire and to keep it from being the weakest link. With the Magic 3 and an excelent ground from the amp to the chassis always seemed to make more of a difference than any Cap until you start getting into very long power cables like in a trunk mounted system in a car. Most systems I have done in trucks usually had power cables under 7' long where some trunk mounted systems were in excess of 14' long.

Right now I'm running 5.5' of 1/0 then very short leads of 4 out to the amps. All my grounds are under 12" with bolts holding them down to bare metal.

 

i would agree witht he above and say there must be somthing wrong with the install.

I run four type r 10's and a tottal of 2100 watts rms in my truck all running off two deep cycle batterys and the optional 130amp factory alt and i have 0 dimming of any kind. as for the 10awg wire thats just silly and i'm shocked they recommend that. i run 0awg to my block and the 4awg out to the amps. you should have NO problems with a your truck and only 300 watts.

as for the 18 inch ground rule. thats 100% correct and good practice. if anything you can go even shorter then that if possible.
the sorter the better.