I have a DC-300 service manual dating back to 1985, and you would not believe then number of part changes and service bulletins concerning the class A driver transistor, and mods to keep this transistor's substitutes stable in older circuit boards. Yes the DC300 is stable, but if it blows up, good luck! Without access to the bulletins, who knows how well the repaired amp will perform. My guess is the DC300a and II versions do not have issues.
I find the DC-300 sound to be thin and screechy. It would probably benefit from higher class AB bias, but the heat sinks (like the Phase Linear 400 4 fin) are too small. A modern Crest or old school Crown macrotech amplifier runs circles around the DC-300. The classic '60s system of Altec A-7 Voice of the Theater speakers and a DC-300 was flawed. The DC-300 seemed to exacerbate the worst qualities of the A-7.