Help need speaker to match Threshold 4000 amp....

But the SP-17's impedance is 60k... I think my Stasis 2 is 15k just like the 4000a

I am confused about this 4000 amp input impedance.

1.My e-mailed responsed fr Jon Soderberg regarding about 4000 ,input impedance is 15k.

2.e-mailed responsed yesterday fr.Nelson Pass,input impedance is 75,000 kohms.

3.One member from this trade :
http://trade.audiasylum.com/ca/ca.html?ca=10701
Input impedance is 75k ohms according to 4000 manuals.

With all my respect mentioned name above I do believed to Jon Soderberg for his top notch P/M & upgrade & advise
Same thing from Nelson Pass information.
Which is which now 15k or 75k ? HELP!pescolar

I apologized for last # 3.Pls.desregard..trade.audiasylum.
It should be.
http://trade.audioasylum.com/ca/ca.html?ca=10701

Nelson Pass designed the thing. I would listen to him.

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.