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

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

Everyone will have their favorite speaker brand. Mine is Tannoy. There are frequently lots of tempting (to me) used Tannoys for sale.

I’ve used Tannoys with lots of VAC components for many years now. In the bigger system I have Canterbury GR with VAC Master monos & preamp. In the office I’ve used Glenair 10’s (I swap these in/out with Canterbury SE), with VAC Statement 450S amp. It’s quite a price disparity, but a great match. I got these Glenairs used from TMR several years ago for $2000. For $3500 you’re not quite to Kensington money, but maybe close-ish (SE). Turnberry SE should be in range (current line is GR; prior SE models are very good). Maybe you could swing modern Legacy Cheviots. Avoid the DC10A, which are frequently heavily discounted - great looking model but I don’t know WHAT the engineers did to that poor speaker (crossover)? There are older 90s / 2000s models which are exceptional for the money too - e.g. D700. And of course the many vintage options, which Tannoy is known for!

It’s either your sound, or not. The other speakers I’ve heard and liked (but not enough to change!) are much more expensive than 3500 even used: Magico S1, S5, Acora, Sopra 3, Von Schweikert VR55-Aktive, Audio Physic (forget the model but it was $14K+ and had a cone tweeter). I didn’t like Magico A3. Nor the really expensive Vons or Focals. The Magico S1, ueven Mk 1 used doesn’t quite get to 3500. Maybe some of the Audio Physics would get there.

@mulveling  +1 on the Tannoy suggestion. I don't own any currently. I worked in an audio shop in the 90's and 2000's that was a dealer. I loved the old Mercury's, always a no brainer for customers. The sixes series were great too, but their look was a bit polarizing. Before I went for the Volti's the Cheviot's were my main contender. 

I am still searching for a good and fair priced pair of Eaton's or System 12's for the mancave. 

Was also going to throw out the Paradigm Founder series to OP. 

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.