Which Threshold s are best?

The stasis design as implemented by Nakamichi is an lower budgeted implementation of the Threshold. I owned the Nak PA5A MKII which was the upgraded 150wpc stasis version with huge heat sinks - very 'cool' looking and operating. I was very happy with it driving my hybrid ribbon speakers. Yet, it was completely unable to drive my estats. So, like any other component, it depends what you match it with.

I believe that the SA series, made in the mid 80's were the best amps made by Threshold when Nelson Pass was in control. Pure Class A. The SA3 a 50 watt model was outstanding and the SA1 mono's are a classic. They still cost 3K+ if you can even find them on the used market. The S500 was a great AB amp if you needed a lot of power.

Threshold SA/1 mono block (pure class A) power amps are the best sounding poer amps that Nelson Pass had designed, period.

Personally owned just about every Threshold amp that was ever made, and still have about two dozen Threshold pieces.
The SA/1's standout alone on top.

As good as many Threshold amps are, they can all benefit from component upgrades / internal modifications.

One of my friends has a very highly modified Stasis 2. He swapped out many of the caps, diodes, etc... of the existing circuitry, increased the filter capacitance of the power supply and then had Jon Soderberg of Vintage Amp go through and align it. While this was the most heavily modified amp that Jon has ever worked on, he also said that it was probably the best sounding Threshold amp that he had ever heard. Given that Jon used to work for Threshold and has probably repaired / modified hundreds upon hundreds of Threshold based products, even after they went out of business, i think that his comments should say something about how much parts quality equates to what we hear. This is just another reason why i encourage others to upgrade / modify what they already have, if they are basically happy with it.

With that in mind, i don't think that you can go wrong with any of the mid to later series Threshold pieces, even in stock form. Some are better than others, but all of them have the potential to sound very nice. While I can't say that i've even come close to hearing or examining every amplifier or design out there, Nelson Pass is my favourite amplifier designer at this point in time. He has been for several years now. Sean
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For class AB, the S500e and 550e are the best I have heard. For class A, I prefer the T400 driven balanced by the T2 preamp. I think this is the best I have heard period. The T400 is basically dual mono unlike the lower power amps in the T series. John S mentioned that the bridge rectifier can fail on the T200 when driven hard over long periods. He has a simple fix and can update the bridge to the fast recovery type. Please note that most preamps cannot handle the low impedance on the balanced inputs very well.