Sub size has nothing to do with sound quality. There are 18's that have a better quick response than 8's. Your bass quality doesn't diminish or get sloppier as the size of subs increase. It's a myth that floats around.
It's not the head units fault. It's the fact that you are trying to run 10 speakers with a head unit designed to run 4. No head unit is going to fix that issue.
This thread contains massive amounts of fail. From the "dvc is made to run off one or two coils" or the "set it with a dmm" has made my brain hurt from multiple parties.