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

As high up in the Vivid Audio G series as I can get G3 or G2, G1 will be out of that range.

My HI-Fi experience comes from the period 1960s to the 1980s, when I had a HI-Fi store in South London. At that time £1000 would have been considered a huge amount to spend on loudspeakers.
My pair of Tandberg Monitors have been with me for more than 50 years and have given me great satisfaction. My system is all Tandberg, including two TD20A tape machines because most of my music originates from reel to reel live recordings of symphony orchestras. 

There are one or two key points that I consider most important regarding speakers and which probably don't get the attention they deserve.

Positioning: particularly if you listen to classical music.

We should bear in mind that in a concert hall situation the bulk of sound derives from centre stage, naturally there is a spread to left and right, depending on your seat in the hall. Therefore placing the loudspeakers extreme left and right, with the bulk of sound firmly placed in the corners, could not possibly recreate the concert hall effect.

Back in my day Lowther, in the UK, manufactured what was known as the Acousta Twin, only psychology made the centre positioned, single cabinet difficult to accept for those, who had been brought up on 'ping pong balls' and 'puff puffs'!  However, for many situations, narrow room, small room, that sort of restriction, the unit was ideal. Here is a link to the the Lowther museum:

http://www.lowthervoigtmuseum.org.uk/lowtherTwinAcousta.html

As you will see the top of the range Twin Acousta cost only £90.00 in those days.

From the same manufacturer a pair of Audiovectors would have been the ultimate for a reasonably to large sized room. Not only that but the design respected the obvious, that a double bass is a large instrument constructed of wood, therefore why would we expect that small shelf speakers could come anywhere near?

The other point is the origin of the sound we use to drive our system. Take this sound sample for example:

https://www.dropbox.com/s/sc3tlxveknhyt04/closing%20bars%20Leonora%20no%203%20Kurt%20Masur%20Hastings.wav?dl=0

I think that, regardless of your choice of music, when the sound is right classical can be enjoyed by all. 

For my ear that is a perfectly natural sound, which demands the very best installation but also should be heard on high quality headphones for the intimate feeling and also for the older generation. Warning, watch out for the dynamic range. The recording was made with absolutely no electronic interference, it was conducted by the conductor. Incidentally I think you will agree that the balance is perfect but will, perhaps, be surprised to hear that only two microphones were used in the recording process.

From a lifetime of living with the symphony orchestra and sound equipment, (I'm 80 by the way), I cannot help wondering if the belief that a huge investment is essential to produce the ultimate results, might be misguided. 

My own belief is that having spent a fortune, even with a custom built room, if the source of sound is not perfect then it is all a myth. Of course, I appreciate, that when it comes to high quality sound source there is very little choice.
I also appreciate that most listen to Rock, heavy metal, film sound tracks etc, which is not so demanding as classical music consequently speaker location may not be so important in such cases. 

I could ramble on but doubt that anyone is interested.

Ciao, ciao,
Geoffrey

Obviously its personal preference.  For me,  I recently bought a used pair of Von Schweikert VR7's off Audiogon with bad veneer for 13,900 and sent them to Von Schweikert Audio to upgrade to MK2 status and have sanded down and custom painted Ford multishift auto color.  I am presently waiting for them to be shipped to me know.  The upgrade was 14K so a hefty investment.  But I have owned VR5's before and they were stunning and I am just dying with anticipation to get these bad boys.  Cheers. 

Given the last several months listening to them, I vote for the Avalon speakers. ...but save some cash for a good cable to bridge your amp to the speakers. These speakers reveal every flaw upstream, so eventually the cables will demand being upgraded ;-)

Sell your amps to add that equity to your funds and buy the biggest ATC's you can afford. Rarely will you see them used since their owners never sell them, one you've reached the top.....?