I just wanted to add to Swift_GTI's review of the 41Hz kit/support.
I have built 2 of the amp9's from 41Hz. I ran into some issues both times; none of which were the fault of 41Hz. A couple were my own stupid boneheaded mistakes. I also had some deep technical support questions as well when constructing the first amp as it pertains to AM radio and automobile use.
Anyway, both times, Jan at 41Hz went above and beyond the call of duty! All my technical questions were answered fully and timely.
As far as kit quality, both amps worked on the first try. They are very easy to build, and the instructions are quite good. The amp9 assembly instructions do need a revision though as there are 2 parts I believe that aren't labeled as to where they go, but with the datasheet from Tripath and process of elimination its very obvious where they go, but really should be documented in the assembly instructions.
Overall, If your looking for a 4 channel amp that is small and powerful enough to replace a headunit or for integration into a carPC, I don't think you will be dissapointed with the kits from 41Hz.