After searching high and low I got my radio fixed. There's a guy named Baris in the UK who repairs BMW radios. I found him on EBAY, his vendor name is carphonicsuk and the item is:

BMW Becker BM53 Radio Professional Upgrade Repair Service 3 5 7 Series X5 | eBay

It cost me $168 to get it repaired plus ~$20 shipping (US Post office).

FYI, UPS wanted $147 to ship radio from USA to UK.

It took about 3 weeks from the time I shipped my radio until I got it back WORKING! No more low volume distortion.

--Jim