Woohoo, finally figured it out! Mike - I tried exactly what you suggested early on, but still no fix. This thing has been annoying me for a week. What I never ended up realizing, is that this thing has a front center speaker. Had I known that, I probably would've figured out the issue much quicker.

Its funny, I was frustrated so started putting various speaker wire combinations together.... at one point I mixed the (+) from the left side and (-) from the front right side, and all of a sudden music came out of the center channel. In the stock harness (brown plug) I always found it odd there were 2 wires into the (+) on one side and (-) on the other, but just assumed it was something to do with the tweeters. In reality, what Audi did was bridge the front center speaker across the right and left channel. Thus the reason my deck was clipping, they were operating at too low of an impedance since you were running the three speakers off of the two channels, with one of them getting essentially bridged.

The simple solution? Snip the two wires that are leading off to the center channel (they're the two smaller wires, one is brown w/green stripe, the other is green w/blue stripe) and tape up. You can easily reconnect these later if you put the stock HU back in.

And phew - it works perfect! Thanks everyone for your suggestions and pointers!