Speaker for Threshold amp and preamp

I have a Threshold T400 power amplifier and T3 preamplifier. I am interested in hearing your opinions on these, and looking for speakers suitable for this system. Thanks for you help.
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.
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.
Well, you've heard some good speakers that you can't be sure that you would like. Whether you like speakers a depends largly on how they integrate with your room and the electronics you use to drive them. So my question, rhetorical if you like, is underwhat circumstances did you hear and not like those speakers you referred to? Personally I'm fond of the Tyler Acoustics Linbrook Systems one piece full range system (lots of post here on Tyler speakers), Spendor 100's, and Paragon Jubilee/Jems (which are very hard to find). I think the Tylers have the best quality/price ratio and for a large speaker are easy to set up in a medium sized room.
Your Threshold amp should be able to drive almost any speaker, they are excellent amps. I find Threshold amps to sound a little on the lean side and seem to sound better with a fuller, slightly warmer sounding speaker. I have a Threshold S350e amp and Fet10e pre myself and was driving a set of Infinity Renaissance 90s for the last 10 years. Although the Threshold amps have high current capability, Infinitys are not the easiest speakers to drive and I always felt that they sounded a little lean and didn't quite have the punch and dynamics I was craving. Last fall I found a deal on a demo pair of JM Lab 946 Electras and man what a difference. They are much more efficient than the Infinitys were, the music now sounds much more alive and dynamic. I have heard people say the JM Lab speakers sound too warm or full but not with my Threshold, perfect match. If you have never heard the JM Lab Electra series before I would give them an audition, almost Utopia quality sound at a fraction of the price. Of coarse this will also depend on your room as well. There is something to be said for efficient speakers though, even with a high current solid state amp. I find the presentation to be relativley neutral with these amps very similar to the Pass Labs X series. I would just audition speakers driven only by a good quality s/state amp and go from there. Hope this helps.
My short list:

Hales T8's
Vandersteen Model 5's
Thiel CS6
Montana's (model #?)
Dunlavey model 4a's

I have heard both the Hales and the Dunlavey's. Both are excellent. The Vandersteen has been remarked as sounding similiar to the T8's.
I have a pair of T400 bridged to mono driving B&W 801 matrix S3. The sound is very powerful, rounded and natural. Voices are very clean and sweet. Highs are clean and defined and lows are powerfull and controled.
My pre is a T2 modified by Jon Soderberg.

Liking a speaker's sound is very personal but one thing I would recommend you is to upgrade to a T2 preamp and get it modified by Jon. The T 2 is an excellent preamp and the modifications take it to a higher level lowering the noise floor, increasing the purity of the sound and the control of the bass.

Hope this help.