After years of living with only a single channel, I attempted a repair and it has been successful so far. Though there is a turn on thump (perhaps because the output devices are not as matched), it seems to work ok.
In one HF channel, the fuse had blown and kept blowing. This was the initial symptom. I suspect the culprit was the original goop used to hold things together. It became conducting and shorted out one output device (TIP41C). The blackening of the board is suggestive of this.
Thanks to the excellent work by the folk listed in the references below, I had an idea of what to do. In the end, I replaced both Q9, Q10 (TIP41C and TIP42C respectively), and Q6 2N3904. TIP41C had all terminals shorted; TIP42C measured ok. I also changed out a couple of resistors that seemed very charred, though they measured ok (They fell apart when removed from the board). There were mica insulators between the output devices and the heatsink. I cleaned the old heatsink compound and applied new compound. The main clue was to compare measurements between the working and non-working channels.
There is an initial offset of about 60mV in the channel I repaired. This is about 20mV in the other channel. They both settle to about 5mV higher in a few minutes. There was some discoloration in the LF amp board but I did not change anything since it works.
References:
http://christopherbradshaw.net/The_Project_Bin/klipsch%20promedia%20subwoofer%20repair%20-%20page%201.html
http://www.thompdale.com/bash_amplifier/2-1/2-1_bash_amp.htm
http://diyaudioprojects.com/Forum/viewtopic.php?t=4892
