I apologized for last # 3.Pls.desregard..trade.audiasylum.
It should be.
It should be.
I'd say you're between a rock (Soderberg) and a mountain (Pass), both extremely knowledgeable about Threshold (understatement), but like Taam I'd have to give Nelson the nod. I was the one who sold the 4000 and posted the specs you linked, Nelson's answer matches the 75k in the manual (I have another 4000 for sale - rebuilt by Jon).
During the almost 30 years I used the 4000 I paired it with an NS-10, an Amber Model 17, an ARC LS-1, and an ARC SP-16 (even a Dyna PAS3x for a little while ... Ha! PAS on Pass). I have fond memories of the NS-10, though I liked the SP16 best, that said there are a lot of good preamps out there, you'll get good advice here.
As for speakers I had Tympani IIID's at first but Beveridge Model 3's most of the time - vertically bi-amped with 2 4000's for the last few years. In the end they were mated with Maggie 20.1R's. As noted they can drive about anything (the Bevs reportedly dropped below an ohm at the high end).
So one mans pairings with a great amp, sorry to go a bit long winded, been a while since I've posted on the 'gone.
05-19-11: Pescolarlooking at the specs only, the Cary SLP-98 looks like a good match. Anyway you can audition it/try it before buying?
Another person I know (who has a Threshold T series amp) uses a Music First Audio (MFA) passive TVC preamp & really likes what he hears.
An obvious match is what some others have already recommended - the Forte (down-market Threshold brand) & Threshold s.s. preamps such as Forte F-44, Model 2 OR Threshold FET-9, FET-10HL, T-2, T-3.
I wanted to clarify my post above. I contacted Nelson about my Stasis 2 and he informed me the following.
The input impedance is 75K. There is a 4.75K resistor and then a 10K resistor, the 10K has been bootstrapped to
a 70K value.
So the 75k reported looks to be the correct answer for both the 4000 and Stasis 2.