Nice mini-theme; really enjoyed it. I'm beginning to reconsider my position on mini-themes — before I've said that I like them once in a while, but I've been more and more appreciating the extra juice they give, goosing up my solving enjoyment. This one was a little kooky in that the B -> B♭ -> Bb6 progression is slightly marred in my eyes because a flat symbol is not equivalent to a lowercase "b," but I admire the audacious approach. A triple stack of this nature is no small feat.
Speaking of no small feats, there's a reason why mirror symmetry is rare in themelesses. Why? Because a basic tenet of many themelesses is to use four separate stacks of 8's, 9's, or 10's, one pushed into each corner. This isn't possible using mirror symmetry — four sets of stacked 7's yes, but anything higher, no — so a constructor must break new ground. Takes a bold person to venture into largely unknown territory.

Some beautiful clues, too. [They might catch some rays] has nothing to do with UV rays — it's referring to manta rays. DOG EAR has a brilliant clue, playing on the "turn over a new leaf" idiom. PRINCES succeed to the throne, not succeed in life goals (although some do both, I suppose). Even the little AVE shines; States Ave. getting its capital letter camouflaged at the beginning of the clue. A lot of cleverness imbued into the cluing today.
Interesting point Will brings up. I lean toward the test-solver who frowny-faced at the LETTER quasi-dupe. On one hand, Will is the editor and gets to make the rules. On the other, the quasi-dupe feels inelegant to me. I realize that's a completely subjective assessment based on my own biases, but it is what it is. Not a broken law by any means; perhaps more of a hitch.
Same goes for the number of three-letter words. I did feel like the solve was slightly choppy, and I'm guessing this was partially due to these little guys. Most of them are fine, but the effect en masse took a little away for me. Similarly, I tend to notice when the crossword glue count gets over about five. Here, the combination of ENOL, ALL BY, A TALE, IN HER, LOLAS, ESA, etc. felt like too much.
So, some compromises, perhaps too many for me, but overall I appreciate Joe's neverending quest to do something different. There's very few people who push the envelope as much as he does.