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

2nd the Paradigm Founders. Really great speakers. Play well with tubes or ss. I have the 120h. Currently paired with a pass xa30.5 and tube preamp they are amazing. Also had them with an EL34 integrated and also very very good. You can find the 100f in your range for sure or the 120h for 4kish used if you look around. 

Used Sonus Faber, Totem or DynAudio (more detailed oriented than the others. The Sonus, best choice given your musical inclination, but not over ten years old. Some of the older ones were too warm and compromised detail. 

I have to agree with @grislybutter... if you could stretch to $5K... you could get a higher level speaker and really advance your system. But used Sonus. 

Spendor A5 or D7

They do so many things right if you lean on a kaleidescopic sound stage, quick burst microdynamics and transients along with refinement and clarity.  Careful setup required but the user is rewarded.  

Tektons (models with the array) have a unique type of sound, in some ways a hybrid of a horn and concentric driver type of sound perhaps....deserves to be in it’s own pocket like magnepans or electrostats or whatever.

For the guy who clicks with that type of sound, some others speakers may sound kinda lame. Hence, it has a following.

Their customer service as well tends to be better than most.

With hundreds of other loudspeaker choices why would anyone want to deal with Tekton? What are you trying to prove?