Another factor you might check is the 600-ohm bias pot setting. The factory manual states "with 14 volts applied and a 10-ohm speaker connected, adjust for 1.5 volts between the collector (DS-501 outer case) and chassis ground". This pot is often corroded, and I've found no continuity between the two major metal parts of this pot, causing a mis-biased condition and lotsa heat.
http://en.wikipedia.org/wiki/Load_line_(electronics)
The pot sets where the transistor operates along the load line. I know this is confusing, but try this analogy:
Think of how you would center a steering shaft in a car so you have the same amount of rack travel when you turn in either direcftion - mis-adjusted, the shaft will top or bottom out after an unequal amout of turning by the operator. The amp is the same way - you want the signal amplification to be centered along this useful area of the transistor - and the bias pot sets the circuit to allow this.
Lotsa luck...