but does Proper borrow? I think that is the question here. The answer is surely yes in some, but certainly not in all cases.
A set like the "Swing Tanzen Verboten!" one, is highly unlikely to be a rip off of other sources, I should think, and a good buy if you want some obscure European music from Nazi times, with an interesting booklet to go with that. Other sets plainly reek of dark business, stealing masters elsewhere, going through the trouble of "touching them up" to make them sound like original, own mastering jobs. Then again I have read (here? elsewhere?) that although a Mosaic rip-off on the surface, judging merely by track title, a certain Proper set turned out to be an original remaster after all; maybe I misread though.
Well, even if they've only "borrowed" once, they're out as far as I'm concerned. Same goes for JSP, Definitive, etc.