Which Threshold s are best?

I have a THRESHOLD SA/20M power without pre but unfortunatly I dont know anything about it. I would be pleased if you help me to find out the answer of some questions:
1-general information about it.
2-which pre is better to match with this power.

The Best Threshold amps designed are the SA Series amps.
These are Pure Class A Designs. And are the top designs of Nelson Pass.

Here are the top 3 Best Sounding and Quality made Threshold power amps. I have had every Threshold amp in the last 25 years.

1) Threshold SA/1 mono blocks with e mod in balance mode.Also love those Big Analog meters !

2)Threshold SA/4E stereo amp in balance mode.

3)Threshold SA/12E (as you know these amps) in balance mode.

I'll buy your Threshold SA/1's. I own a Threshold SA/4e and love it ! I prefer it over Nelson Pass's new stuff from Pass Labs. Had a Pass X350 and even tried the Pass XA100 and still prefer my Threshold SA/4e / But I am serious Mfslgoldcd, I will buy your Threshold SA/1's if willing to part with.

Lawyerman, I'm very interested in why you still prefer the older SA/4e amp. In what ways do you feel it's better than the newer Pass amp designs?

The Threshold SA/4e power amp is more refined than the newer Pass amps. I found has a better midrange and imaging than the PASS amps. The PASS X350 had more bass but the drawback was not as smooth , or natural in the mids as the Threshold SA/4e amp.

True the Threshold SA/4e is a pure class A sound verse the Pass X350 is Class A/AB sound.

Now when compared to the Pass XA100 amp I still prefered my Threshold SA/4e. Better dynamics and still smoother in the mids and top end with the Threshold SA/4e amp.

The XA100 had a slight edge in depth . It seems that the newer PASS amps emphasize more on bass- boom sound than being more dynamic and smooth. In my opinion the Threshold SA/4e is overll a better sound than the Pass X350 . And it's a matter of taste if you want more boom in your bass with the Pass XA100.

I also own a pair of Pass X600 amps which I use in anothe system. Again a lot of bass and depth but misses on the
Mids and TReble smoothness when compared to the Threshold SA/4e. I still like the Pass X600 amps for my application needing bass.

In summary the Threshold SA/4e is a finesse sounding amp. Very smooth, articulate and natural sounding. Still has plenty of power to drive just about any speaker out there even though it's a modest 100 watts pure class A sound. In fact when I had the Pass X350 verse the Threshold SA/4e listening levels were almost the same on my preamp.