UPDATE: Success!!! It’s now working fine and disturbing the neighborhood.
Seems it was most likely Q1, Q2, but I can’t be sure because I replaced several transistors and all electrolytics on the board. New power supply caps too.
Many thanks to the group here for putting up with my dumb questions, and especially to Lee for providing parts and patiently coaching me as I fumbled my way through this.
I’ve learned a lot over these last several weeks, and I’ve acquired a few new tools, so I’m already feeling the temptation to find another “project”.
I’m thinking my Bose active EQ might need to be refreshed...