Restoring vintage H/K Citation 19 Power Amp

I have one of the nicest products from Harman's "golden age" - a Citation 19 power amp, purchased back in '78. Weighs about 40 pounds and still works, though after all these years it probably merits a restoration. Sound isn't quite as tight as I'd like and if I leave it on continuously it exhibits a low level hum (in the 300 Hz range, not 60 Hz). But if I power off and disconnect the preamp, the hum is gone when I power back on.

I'm wondering if any readers have experience restoring these amps, and have some guidance to offer. I don't trust the local service shops to tackle this - one did a mediocre job on a Denon CD deck, the other overcharged me repairing an Integra HT receiver. I know Blue Sky used to do such work - and they were expensive - but have since retired. I have a copy of the Service Manual, but have never tackled a job like this, and would probably mess it up.