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.
