I decided to diagnose the failure on my own. Both Mitsubishi RD16HHF1 PA transistors show a hard gate-to-source short. I ordered several from RF Parts in California (USD $4.25 ea.) and got extras as back-ups. Access to the transistors is very easy. The only caveat is that the source lead is heat-sinked and requires extra heat to remove.
If anyone attempts this repair on their own, be careful to: (1) apply only enough heat to remove solder; and (2) only bend leads when solder is completely molten. Otherwise, you risk lifting the delicate PC board trace. I would not attempt to cut the transistor leads in this case. The cutting shock will also lift the PC board trace. If that happens, you will be extremely unhappy.
I will report back after the transistors arrive and are installed in my SunSDR2-Pro. Hopefully, the driver transistors have not also failed.
Paul, W9AC