If you have the shaker 500/1000 and the door subs, than skip putting an amp on your speakers. I've got 2x amps in mine currently. A 4 channel amp for the speakers, and a 2 channel amp for the door subs, and I notice no change over no amp on the speakers. The only reason I even have it in there is because it was free. I've been tempted to take one of them out and put it in my truck as it doesn't have subs. (yet.) That or get a shaker 1000 box and hook up some bigger subs in the trunk. Decisions decisions.
As for companies, I've always been a fan of Pioneer products, with a hatred for Sony car products. I think Kicker is overrated, but is still a decent product.
07 Mustang GT Metallic Alloy.
FRPP CAI. SLP Loudmouths.
Roush Front Bumper w/ chin spoiler, Roush Hood scoop, Scott Drake Hood pins, Roush 1/4 window louvers, Billet fuel door.