If your just looking to enhange the studio on wheels experience, withoutchanging out your hear unit and front speaker setup and spending a fortune; Keep it simple.
Switch out the rear door speakers and use a 2 channel apm 75watts x 2 min. I used JBL and I think the make a good product.
You won't win any contests for sound or modifications, but you will improve the system and make it bearable to listen to.
