Slightly different, but I built my Hafler 500 in 1975 from their kit. Took it in to the local service guy who fixes everything and it measured perfectly except for a small bias adjustment. Advised not to mess with it, and it sounds as good as it ever did.
Suggest you take it in and see what the tech says. Components do deteriorate as they age, but so far, so good for me, luckily.
Cheers!