So, has the JBL GTO638 been replaced by the GT-6? I'm also trying to figure out if the rear speakers are 4" or 5" co-ax speakers. I found a Polk Audio 5" coax for marine use that looks to be a good upgrade, also looking at PA 4" DB402 if that is the appropriate size. I don't foresee doing an amp, just want to "upgrade" the sound I already have. Thanks!