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

Kind of hard to recommend speakers when you don't say what you have now, what you like about what you have, and what they are lacking. Also the room size, placement, listening volume, music type and what you expect out of new speakers. 

You only have 60w, that might not work with a lot of speakers if you like high volume. That also gets rid of any electrostatic speaker. 

Also, if you are very happy with your current speakers, since they are over 20yo, why not just referb the crossovers? It will be cheap, and they might sound better then new after. 

Well, after 25 years I might want to upgrade even if I still like the speakers.

Rooms will be changing, that's not part of the equation here, they should sound good in any but very small or very big room.

mulveling,

I see. I agree, coherence is very important, single driver speaker without flaws is an almost unachievable goal.

Yes, choosing Tannoy would be a very complicated matter, especially if you can't audition them. I also understand that the better ones will go over $5k used .

@inna 

Given the limited amount of power you have on tap, would a good pair of bookshelf speakers paired with powered subwoofers make sense in your environment?

That would make things more complicated and probably more expensive. Integrating subwoofers can be a challenge. I like coherence. But theoretically yes, I have enough room for that.