I had tons of bass in mine, but I had the infinity system.

 

From what I remember when I swapped mine I think 6x9's would be a very tight squeeze back there, you'd definitely be doing some cutting. As for 6.5" speakers that drop in I put Polk db650's in mine and they fit fine. I wouldn't mind deeper/more bass either but that will have to wait for more stereo upgrades and a sub.

 

I looked at a Ram 1500 Outdoorsman last year with its highly touted interior storage boxes. The Factory "engineers" used the largest storage compartment under the rear seat to house one of the speakers!

 

The tweeters on mine mustve been too tall to fit the plastic panel them. Hmm. Well, I'll try them again one day.