Re: Variable sound quality, JBL PRX rig
+1.
I used to own a DRPA. If you want to see a perfect example of comb filtering, run the auto-eq with both sides of the speakers turned on. Go to the eq page when it is done and see what it came up with. Looks just like a comb 
I found that the auto-eq on the DRPA (not plus) tended to under emphasize the LF to my ear. It also left the mid highs too hot which results in a brittle sound to the PA.
Give this a try. Take the DRPA out of the loop. Run directly from your mixer to the XLF subs inputs, then run from the XLF outputs directly into the 615's inputs. Leave the 615's eq curve on "mains" not "monitor", and ensure that the XLF's have their cross-over turned on.
You are correct. Running the system like this eliminates any holes in the frequency spectrum. Furthermore, this is how the designers at JBL designed the system to be run.
I don't know many sound pros that like what the auto-eq does to the sound.
