I am the original owner.
Recently performed was a complete check-up by Audio Research. All four tubes in the power supply were replaced although only one 6550 was in need of replacement. Tubes in the pre-amp unit tested good (1335 hours). In addition a Zener protection mod was added to both channels of the power supply to improve reliability. This is in line with improvements found in the REF 10 pre-amp. A 24-hour burn-in by Audio Research was performed to assure all bench tests were passed.
I have owned several Audio Research pre-amps beginning with the SP-3a in the 1970's.
This pre-amp is a marvel.