Pass Labs AMP’s- What is the best Pass Amp Sound for You from past 10 years

@kosst_amojan Definitely, that's a win. Congrats!

Over the past decade I've used or demo'd a variety of Pass products during my speaker/amp journey...  Thiel 2.4s (Classe CAP-151, McIntosh MC402, Pass X250.5, Pass XA30.5... eventually settled on a Pass INT-30A), Coincident Super Eclipse IIIs (Pass INT-30A, First Watt SIT2... eventually settled on Coincident Frankenstein 300b Monos), & Coincident Pure Reference Extremes (Coincident Franks, Lyndgdorf TDAI-3400, Atma-Sphere M60 Monos, Pass Labs XA25... eventually settled on First Watt F4 Monos).  

As pure current sources with no voltage gain, the F4s are clearly not for everyone/everyspeaker, but they excel on my PREs.  As I have pointed out elsewhere, the 6moons summary description is pretty spot on.... "relaxed ultra resolution."  They have a bit of the sweetness of my 300b SET amps, but both clarity and bass control that beats out all the other amps I've auditioned on these speakers.  The soundstage is expansive, imaging is precise, and there is absolutely no harshness or grain... an important quality with the accuton ceramic tweeters on the PREs.  

I got my first F4 (actually I built it) to serve as a zero-gain bass amp to augment my Coincident Franks, but unfortunately biamping on the PREs just kills their coherency (I've tried both active, with the Lygdorf, and passive, with the F4)... even with the Franks providing the F4's input signal from their speaker posts.  When I finally ran the F4 alone I was shocked to find that it was better than all the other amps I'd tried with the PREs. 

Previously, the Franks (currently up for sale) had won every shootout, but they simply don't have enough bass control for the PREs and I was forced to move on.  Super happy that I stumbled onto this amazing speaker/amp synergy, however. 

Thanks Nelson, for coming up for the design, and making it possible for people like me to build their own (along with help from the amazing community over at diyaudio) now that the amp is no longer being produced.

@ajant I have Coincident Pure Reference Extreme speakers.  I think they're around 92db/1w/1m (listed as 94db, but I don't believe it) and supposedly a flat 8ohm impedance. 

I'm sure you could get by with a single F4, mine worked fine that way... but I still believe in monoblocks for isolating right and left channels.  I have tried my F4 monoblocks in a bridged design and actually preferred just using a single channel.  Essentially you're doubling your power supply capacity that way.  I found that there was less distortion with an increase in clarity, smoothness, and sweetness from the amps using them in that configuration.  And yes, I find the amps very nicely 3-dimensional, at least as long as the source and preamp support it.  That's a key decisional factor for me for every component.

I've used two preamps with the F4s, a Bob Sachs 6sn7 pre and a custom Shiny Eyes EML20A DHT.  The Shiny Eyes pre is a simple single stage single tube DHT design... from Radu Tarta: "This is one of the simplest one stage preamps that is using an excellent sounding DHT and extremely low distortions, second harmonic being dominant. The gain is 14dB (5x) and with a 2Vrms source will provide 10Vrms at a Zout of 375 ohms. Measured THD was 0.02% at 10Vrms output (50Vrms on the plate)."

It gets plenty loud enough, but I do periodically wish I could be idling a little further down the volume control ladder.  

You can read more about my amps, speakers, and preamp on my Audiogon system page.  I personally couldn't recommend the F4s more based on all the amp comparisons I've done with my speakers.  I actually ended up rebuilding them with custom high quality parts and massively overbuilt power supplies (so now 4x the levels of a standard F4 for each channel) and they just got better and better every step of the way.  That information is all outlined on the system page if you happen to care.  I would only change amplifiers if I ended up with truly high efficiency speakers over 100db/w/m, or needed something with a lower damping factor.

Hope that all helps.