Thanks, Paul.
Just to have this info in a more recent topic, here's the contact info for United Radio for Mark Levinson amp repair:
website: www.unitedradio.com (still active)
Automotive Electronics
5705 Enterprise Parkway
East Syracuse, NY 13057
Toll Free: 800-448-0944
Phone: 315-446-5570
Toll Free Fax: 800-701-3874
Fax: 315-446-2434
A topic in 2011 indicated that a repair cost about $325.