Hi ctuna, thanks very much for the help! I will post some photos of my setup and wiring once everything is settled, hopefully that will be my small contribution back to the community!
I figured out what was making the rattling noises on my driver's underseat. There was a little piece of broken plastic (like a small piece of a ziptie) bouncing about on the cone, causing rattling noises even at low gains at low volumes. I removed the debris and turned up the gains and the lows are sounding incredible!
One question though, on your photo, for Ch 5 & 6, it looks like you've got the gain for the underseats turned up to the max. Is this correct? I can't really get past 6 o'clock (about halfway through full travel) without everything sounding too boomy.
Also, the final issue I seem to be having is the hiss/white noise coming through the midrange and especially the tweeter. It's faint when the gain for Ch 1 & 2 is set to min at 1 o'clock, but when I turn it up even a little, say to 2 or 3 o'clock, it gets somewhat noticeable when music fades in or out or when I'm listening to pure vocals. I have plugged out my rca inputs into the amp and the hiss vanishes. My input wiring is set up as follows:
OEM Headunit --> OEM wiring --> Technic Harness --> RCA male-male extension --> XD600/6
Anything here that could possible induce hiss or noise into the system?