actually, easier test. Turn the bit one master down, and crank the HU to full. If you hear the distortion with the HU full and bit turned down, then you'll know its the headunit. If you don't, than its something downstream. Either the bit is clipping/distorting, or the amps.

 

Just happened to see your thread when I was lurking around for some info on my next project. As it happens, I did a build in my car (an 07 sedan) that's pretty similar and posted a full build log on DIYMA. It may hold some interest to you:
Charliekwin's 2007 G35 Sedan build log - DIYMA Car Audio Forum

Obviously I'm not a regular around here, but if you have any questions, I'll keep an eye on this thread for a bit. You can also hit me up on DIYMA.

 

Great DIY skills! Kudos

 

Nice blog! Ive learned a lot!

As promised here is a short run down of what Ive done so far!

1. Ran pretty good quality wiring from the HU to the trunk.
2. Installed Morel Elate 3 ways in the front doors.
3. Dynamated the doors, trunk, and back deck/seatback.
4. Made a temporary Amp rack with an Alpine 4 channel running the Morels and a used Soundstream running my sub until I replace it with the matching Alpine monoamp.
5. Mounted the Audison Bit One where the rear deck 10 inch sub was.
6. Put a 8 inch Digital Designs box in the back until I get my JL 10w6.
7. Ran fiber from the Bit One to the front and using an Astell and Kern AK100 MP3 player with Wolfson processor and optical out.
8. Had my tweeters modified to pull them out from the A-pillar and added Oil condensers (signal capacitors) to the tweeters to smooth out the sound and make the highs clearer (I was surprised how much of a difference this made in the clarity of the highs!)

Sorry I only have some photos of the tweeter mod and oil condenser I snapped on my iphone, but I'll upload more soon!