Brit - sorry to hear of your troubles with the buzzing. The fact that the buzzes with the throttle indicates you have a ground loop gremlin in your system. The xm7's are a very efficient speaker , so any noise issues in a given setup will be heard on then first.
There are several things to take a look at as We isolate different areas to pinpoint what's going on and from there make some changes. First thing I'd like you to do is bypass the 420 and plug your iPod directly in to your amplifier.
After you do that, give me a call and I'll walk you thru the 12 or so other checks that may be needed. Whatever the case , I'll get to the bottom if it for you.
I can be reached over the weekend if need be.
Last edited by brianinpdx; 08-13-2010 at 10:39 PM.