What used speaker would you buy with $20,000-$35,000 and why?

Used Wilson MAXXs

Al, this is what Cardas has to say about the golden cuboid (yes I left the word "golden" off earlier - saving time):

"The Golden Cuboid listening room is 10’ x 16’ x 26’ (Read 10.000 x 16.18033989... x 26.18033989...

It’s dimensions differ in a Golden Ratio or Fibonacci sequence (5-8-13-21-34...)."
From: http://www.cardas.com/room_setup_golden_cuboid.php

My room is 10' x 16' x 26' but is not not expanding in dimension as one moves out from the short side of rectangular room. I didn't design the room that way. It's the size of the front room when we bought this house (hence the need for some structural support - the floor joists are not sufficiently engineered for a really tight/stiff 16 foot span).

Good!

A lot of good speaker recommendations have been offered, which would be suitable for a room of that size (assuming, in some cases, that the structural improvements you are intending to make would enable several hundred pounds per speaker to be comfortably supported).

In addition to the speakers that have been mentioned, you might want to consider some of the models from Sound Lab. One of their dealers, btw, (for the Rocky Mountain region) is Duke Lejeune of Audio Kinesis, who participates here from time to time as member Audiokinesis, and who is by all accounts a wonderful and very knowledgeable person to deal with.

Disclaimer: I have no personal experience with Sound Lab speakers, but if I were shopping in your price range, and had a room of your size, based on what I know of them they would be high on my short list.

Good luck. Regards,
-- Al

None, the better bang for the buck is to go surround 5 speaker setup with cheaper speakers. It’ll sound better too.