I enjoy my 6230 a lot also. I also got it off ebay, about the same price shipped, ( under 150) and like yours, had some problems. It hummed like yours, and one channel would drop out, and come back it you jiggled it.
If you take the top off, you'll usually see where someone has added a wire, I inquired about it on mine here and was told it was anextra ground wire many added to reduce hum.
I had to basically reflow most solder joints.....These things have run fro hours on end and the transformer vibration slowly breaks them down over the years.
All the output devices, and so forth. If you search on this forum you will also found that there are three small value caps on the board associated with regulators I believe, and they stand right next to these toasty devices and get backed to death.
After all this, you want to see what kind of DC is coming out of the outputs. The power supply caps are remarkably durable, mine were still fine. Has worked fine ever since. This is basically a tank of an amp, its output devices are only being used at a fraction of their capacity. Once together, it should run fine another 25 years or so.