And as if he knew I was planning to cause such turmoil, suddenly the male Saturday began singing -- for the 1st time in at least 6 weeks or so! At first it began quietly....I wasn't even sure I heard it. Then he gradually began piping out a real chorus for the next 20-30 minutes. I can assure you -- after so much concern these past few days, his little song was like hearing Tchaikovsky!

So I'm assuming whatever was preventing him from singing these past couple of months wasn't air-sac mites, or he wouldn't suddenly rebound....right? Frustration over the inability to nest seems to be the most likely culprit after all, in this case. Apparently he's just given up. (?) I still may yet treat them both with SCATT in the coming days -- for preventive reasons.
Thanks again for all the guidance; I've learned a lot from this ordeal, and it will make me a much better bird keeper going forward!