It's always challenging for the borderline bands. I can offer no better than criteria then "if it sounds prog enough to my ears I add it". Very subjective obviously, probably inconsistent as well. But that's the way it is.
Suggestions are ALWAYS welcomed, even if I don't act on them. Allman Bros is not a bad suggestion as I've considered them multiple times, but something about their sound is too bluesy or something to be prog. To MY ears, and let me emphasize that this is NOT a bad thing. I am a very big Allman Brothers fan and have a bunch of their music.
Sounds like some Allman tracks could find a one time home in a certain corner of the Mooniverse...
