Wow, tough room. You might want to consult with a pro.
A sheer curtain is transparent to sound, so will not matter.
"If the reference bookshelf speakers just blow the RVX-42's away" - your best bet here would be to find a dealer with both, and listen to them. For bookshelves, I like/own the RB-61's because they are front ported & easier to mount on the wall than the smaller Klipsch bookshelves which are rear ported. You give up a lot going with a "designer" speaker. The RXV-42's -3db point is 92hz, the RB-61's is 43hz, and it's stretching it to try & make up that difference with a sub. Listen to both, & see if you're willing to give up the lack of mid-bass.