I have not pinpointed the problem and have been just living with it or not listening to the radio very much. I have noticed the the overall quality of sound is degrading further even when the fuzzy rumble is not occurring. A good friend of mine that has extensive car audio experience says that I probably have a capacitor or capacitors that are going out. They could be in either the head unit, which probably has a pre-amp built in. Or could be in the main amplifier. Sounds like you eliminated that option. Given that it is volume level oriented, it is probably the head unit/pre amp.
I have looked for head unit options and just have not wanted to spend for the OE replacement. I also have issued with my NAV screen. I'm considering a change to an aftermarket head unit, but have yet to find one that will tie in with the steering wheel controls or that I really like.
If you find any viable options, I would be most interested to hear.