Speaker for Threshold amp and preamp

This combo will drive most reasonable speakers. The questions for you are - How big is your room? What type of music do you listen to? How loud do you listen? How much time and effort are you willing to spend on proper set up? Do you have any sonic preferences? And, what is your budget? With answers to those questions you should get some meaningful recommendations.

My room is about 6m x 3m. I listen to all kinds of music - jazz, classical, rock. The volume I listen to depends on my mood and on the type of music I am hearing. I am willing to spend all the time in the world for proper set up. As for preferences, I don't have too much experience, but I have heard Vienna Acoustics Beethoven speakers, and Plenius, Wilson, Montana, and I find it hard to decide because each one has something different that makes it special. I have Krix speakers at home. I am looking for second hand in good condition. My budget is $4000 - $5000 but I'm flexible if I can find a good deal.

Look into a pair of Thiel CS2.4's

I would look at electrostats from innersound or martin logan.Eminent Technology is also a good choice.

I would look at magnapan 1.6 or 3.6 speakers or soundlab electrostats if you want to go the dipolar planar route. Otherwise you might want to take a look at Hyperion speakers (great reviews). I don't know what the future of innersound will be without its founder, who I believe left the company, so while their speakers (& electronics) were superb future service etc may be a problem. I always thought that the innersound electrostats and the soundlab electrostats out performed martin logan's offerings but that is just my particular preference as ML still makes very fine speakers and is a company that should be around for some time.