$3500 floor standing speakers used. What would be your choice ?

Try finding some JBL on Craiglist you won't be sorry

You could buy a new pair of Monitor Audio Silver 500 or 300 floor standing speakers with that budget.

You have some good options (some already mentioned - ATC’s, Magnepan, Vandersteen, etc.) - but - my choice would be (was) - the following:

1. Linkwitz Orions or LX521s - nothing compares to Linkwitz open baffle speakers (is any better). You can find used Orions for <$5,000 - sometimes for <$4000. LX521’s sometimes for <$5000. And... that’s with the amplifier, the ASP and cables. Believe me - once you’ve heard these in good room - you will find nothing better - and - your search will end for the rest of your life. As noted below: "Linkwitz speakers are known for their realism and 3D sound."

2. Magnepan LRS+ - which comes close to the Linkwitz speakers - again - very few others compare. You can get these new for $1000. And with the right equipment almost nothing is better. The great benefit of these is - that they’re very light weight and are easy to move about to optimize their sound in your room, as well as move out of the way, when not in use. Yet - you still have "World-class" sound... and... you can get a pair for "pocket-change."

3. And... if you’re willing to compromise with a superb stand-mount speaker - almost nothing compares to the Legacy Audio Calibres - which sounds as good as most floor standing speakers and is much easier to manage. Though these are a bit more expensive (around $6500 new).

Got all of these and more - and they are all superb. If you’re an acoustic music fan (jazz, classical, blues, country, blue grass, etc. - real music) all of these are great! If you’re into other sounds - the Linkwitz, or Legacy speakers might suit you better - with a wee bit better bass and dynamics than the Maggies.

Of the others - my choice would prolly be the Vandersteens, because - like the Maggies they’re virtually an open baffle, dipole speaker - which is superb with "all things acoustic."

Because all of these are open baffle, or hybrid open baffle, speakers - as noted - virtually nothing else compares. Far superior soundstage, image, and "life-like" sound.  Very much like their ancestor - the Quad ESL 57's - which is the "king of the hill," since the 50's/60's.  

But, you will find nothing better for <$10,000+++).

Some educational links - say it all:

"Vandersteen Audio and Linkwitz Lab are both companies that design and make loudspeakers. Vandersteen speakers are known for their natural sound and dimensional purity, while Linkwitz speakers are known for their realism and 3D sound."

https://www.google.com/search?q=Vandersteen+linkwitz+audio+lab&oq=Vandersteen+linkwitz+audio+lab&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIHCAEQIRigATIHCAIQIRigATIHCAMQIRiPAtIBCDk1MzJqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8

https://www.linkwitzlab.com/

You can Harbeth 30.2's with used stands for around 3500 and they are absolutely amazing--have owned speakers that are twice as expensive that were no where near as good

@mulveling - I am genuinely curious about your post concerning phase errors in multi-driver speakers. I was reading Floyd Toole's book on Sound reproduction and he makes a point in that book that phase errors in normal full range systems are not usually bad enough that it has a noticeable impact on the sound quality. 

I am not good to explain this, as in the end it really boils down to "I like what I hear with Tannoys". Phase coherence (at the crossover) is only a theory or possibility for why some of us particularly gravitate  to Tannoy DC sound - Tannoys are NOT time aligned, and only phase coherent at the crossover point, but this may be enough? Its symmetric, controlled dispersion is another possibility - this can reduce the impact of a room's acoustics on resulting sound. I also like a "mid-field" arrangement with my Tannoys, which reduces the room's impact even further. With large multi-driver speakers you cannot get so close or the image "pulls apart"! Tannoy's image is clean and lifelike at almost any distance. The supertweeters can sacrifice a little bit of this coherence, but that's another topic and set of pros/cons. 

The key result is that when I listen to something "busy" like hard rock or metal, it has to maintain the musicalilty and a natural "flow", without feeling like it's falling apart or the musical elements are fighting each other. When I do like other companies' speakers, they tend to be simpler, smaller arrangements like a 2-way with the tweeter and smaller woofer close together. An example would be than Magico S1 and Acora SRC-1. These have great, liquid musical "flow" and I really enjoyed listening to them. Sure, I'd love if they had better bass extension and impact, but when you go to larger models - I hated the A3 and did not really enjoy the larger Acoras (including the $200K one) that much. That A3 in particular sounded like it had a Home Theater sub in it (thumpy, out of time, easy to localize), and I hate that - this always punches me out of being able to enjoy music. Yes, room interactions can cause this but I've never heard anything at all like this from a good Tannoy model (to be sure, some are better than others). I did like the S5 (heard it back when it was an Mk 1), but still the S1 may have been more musical. 

phase errors in normal full range systems are not usually bad enough that it has a noticeable impact on the sound quality. 

"Usually" leaves a lot of gray area, and we are all individuals to "some" extent :)