Definately check out Elite Car Audio, the forums there are awesome. Sound Domain is alright, my biggest complaint is the whole site always seems god awful slow. Another favorite of mine is
www.termpro.com The forums there are great and the people are very knowledgeable. It's run by Wayne Harris who developed the Rockford Fosgate Symmetry system and is the founder of DB Drag Racing. It's more geared towards competition but all in all it's one of the better sites.
[addsig]