Jeff,
I know this has to be really frustrating…to get it working and then it stops again. I’m sorry has been such a long battle.
When you hear the high frequencies muffled…this particular symptom is caused because the signal is not making it through the “high” side circuitry. Please check the high side trim pot by working through appendix A, step a, v and vi (don’t bother with the previous steps). I would even start with the “wiggle test.” It’s an intermittent issue and I’ve experienced the trim pots can sometimes be intermittent. While the high side isn’t working, you can also check to see if there is around 1.8V on the optocoupler pin, as in the voltage chart. This will tell us if the trim pot is actually the issue.
Let me know what you find.
-Brach
Ps…I’ll let you know if and when I’m able to accept service work again.