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

Well, indirectly I think I did when you mentioned what you were looking for.  But to be more direct, the Tekton line is noted for a very smooth, detailed midrange that draws you into the music.  Contrary to one comment, these speakers are not bright.  I am intolerant of bright/forward speakers and I can listen all day to my Double Impacts without any fatigue.  Bass extension is quite flat to 30 Hz.  Powerful, taught, extended.  Last, but not least, the imaging is precise with a large, realistic sound stage that exceeds the Magnepans I had prior to the DI's. 

Revel F208 are nice nearly full range speakers and reasonably sensitive.

Might want to look at Vandersteen Model 3A’s.

Knew they’re about $7,190. You may find more recent used, for maybe half that or less. 
Best of luck to you!

Revel plus one ... you cannot go wrong

The sky is the limit and there are so many used speakers to choose from it would be hard to listen to them all. I like Vandersteen and Ohm. For rock and your tube amp, the Tektons would certainly match well. A used or new Salk would be on my list too. Magnepan may be too hard on your tube amp, I know they were on mine. Perhaps look at Emminent Technology?