The dimming comes from the sub pulling so much juice so fast. The smaller and lower power the amp the less juice it pulls all at once.
Components wise, I'd rather spend my money on a good set of component speakers than two coaxials. I'm looking to do JBL MS60C powered by an Alpine KTP-440U (out lined in
http://www.ft86club.com/forums/showthread.php?t=42239). This allows you to use smaller mono amp specifically for the subwoofer instead of a larger multiple channel amp.