Floorstanding speaker experiences/recommendations in the $12-25k range

It kind of depends on the flavor of sound you are looking for.  There are those more on the analytical side and others that are not.  Also, are you looking to stay with a box type of speaker or open baffle design.  I listened to about 12 different speakers before I decided to buy the Vienna Accoutics Liszt model (including some of the manufactures mentioned).  I don't need a sub since the speaker goes down to 28 hz.  I listened to older Magicos, Wilson, B&W, Rockport, Vandersteen etc.  My room size is very similar to yours and they are worth a listen.

Daedalus Audio line may be of interest. I've read only great reviews about them, however I haven't heard them. There used to be a buzz in the forums about their speakers.

You have a cool system, and the Octave has more guts than most tube amps, particularly with the black box.  I know one dealer who has paired it with Wilson Sabrinas and really likes the combo.  Re speakers, I heard some Rethm Trishnas at Capital Audio Fest - only $10k! - and they were shockingly good.  For your size room, you’d probably want to go up to at least to the Maargas, which are still only $13k!  Bear in mind that I find myself increasingly drawn to single driver style speakers, or variants thereto, as they seem to capture the intimacy of recordings better than inefficient designs (to my ears of course). And with tubes and vinyl? oh my!

You should listen to used Vandersteen Quatro wood CT’s , or for slightly larger speakers the 5 CT’s. The 5 s were just replaced by the Kintos. So their price on the used 5s have gone down.  I have the quatros and they are great. 

Another vote for the Joseph Audio Perspectives Graphene. They are a good match to your Rega and more importantly, I have found them to be both revealing and "non-tiring" over long listening sessions.