Great, thanks, couldn't see it was blue. Better change my link colors.
Based on that article, I clicked the Files under Audio, then selected all 7900 files, then selected Stacks and Autostack on Artist, Album, Track, Name. I then asked it to select m4a as the top of the stack.
It seems to have done that. (Also I know I can make MC display only flac and m4a with the check boxes, and probably with other view things that I've not yet discovered. But suppose I want to do more stacking.
If a track has m4a, flac, mp3, or m4a, mp3, the above stacking will work fine.
If, however, it has flac, mp3, it'll still build a stack but there's no direct control over what's on top. Ah, but the stacks page says it'll pick the largest file. So that means if there's all three, it'll pick m4a unconditionally, if there's m4a / flac, ditto, but if there's flac / mp3 it'll pick mp3 because it's larger.
Do I gots it? If not, correct me, please. If so, thanks!