SET amplifiers do not have a singular sound so conversations about them can be difficult. I have lived in Hong Kong a mecca of audio gear. A dealer I often go to is an Audio Note dealer - arguably the king of SET amplifiers because they make more of them than anyone else. As such, I have been able to make direct comparisons. Audio Note uses various levels so level 3 amplifiers are Single Ended Triode and within that level are various SET amplifiers that use 2a3, 300B, 45, 211 output tubes. They all sound different from each other. So which you like will more or less depend on your taste and what you listen to and how sensitive your speakers are. The least of them I would probably take over any comparably priced Solid State amplifier. The Meishu Tonmeister 300B integrated I would take over any SS integrated from anyone. Albeit it's not cheap. Some other 300B SET amps have been described by dealers here as "Lady-Like" in that they soften the sound somewhat but this typically depends on the quality of the transformers. So 300B SET amps have a reputation for being good midrange amplifiers for singers and acoustics but not particularly good with rock and roll. Stereotypes like this get spread around on forums but again it's a lack of exposure IMO to better SET amplifiers. Audio Note at shows does try to ameliorate the stereotype by playing Nightwish, Rage Against the Machine, DIO, various Trance music at very stupid levels to illustrate what SET amps can really do with a decently efficient/sensitive speaker. The Meishu has a lot of Dynamic drive and bass depth and whack but a competing Cary or Line Magnetic 300B does not. They sound "Lady-like" and there are more soft SET amps out there than not.
As for the technology arguments - well SETs are not going to win - the measurements are geared for class A/B SS amplifiers - they test the amps at near full power - where SS measures their best and where SETs measure their worst - in other words, the tests are geared to make SS look better. SET amplifiers have linear distortion which means when you turn the volume up and up and up the distortion rises up and up and up. SS amplifiers often have distortion that is worse when the volume is in the tiny fractions of a watt and lower at 70-80% full power. So you often note that people say that they need to turn the system up for it to "come alive" and that is because the SS amp needs to be putting out some power - and combining them with low sensitive speakers that need power to also "come alive" you have certain systems that really only sound any good when played loud. The other thing to note is that a lot of measurements of amplifiers are actually never measuring a real input. They say see how good the distortion is 0.00001% but they are comparing the input only after the feedback loop has engaged. So the signal is not really the true unadulterated input but and sort of "fake input" with feedback to smooth out the actual input. Possibly why I was so shocked when I heard a Bryston Preamp and Power amp combination with phenomenal spec sheet numbers sound so muddy compared to a 10 watt SEP amplifier. I was there to buy the Bryston with the 20-year warranty and the bombproof measurements and 160 watts of power etc and I had to keep turning it UP to make things out and it sounded bass light and raggedy - the 10 Watt SEP amplifier (which only measures 4.2 watts per channel undistorted was clean and full bodies at low volumes not needing to be turned up to make things out and had better bass. Quiet in terms of noise floor as well. I gotta say back in 2003 it was a tough decision - a 10-watt amp with valves and a 2-year warranty from largely a no-name brand at the time over the 20-year warranty Bryston separates. I chose the SEP. And now in 2021, I can sell the amplifier for several hundred dollars more than I paid for it which can not be said about the Brystons.
The only technical arguments I have really seen for SET amplifiers are their simplicity and lack of error-correcting after the fact fixes (because the amps create errors in the first place and then need to be fixed). Logically that would affect the time domain.
From Audio Note (note they make SET amps so they have a bias here obviously)
"... all-transistor amplifiers sound poor for the simple reason that transistors are inferior amplifying devices. The word “semiconductor” really means what it says and it says it all, “half”-conductor, sonically this could be translated to mean half the signal! Which is really what it sounds like. Pure and simple, transistors are highly un-linear and need a lot of correction (feedback of some sort) to have a bandwidth wide enough to be able to reproduce any music signals, they are not natural voltage amplifiers. Likewise both the pentode or tetrode requires corrective feedback to lower the load sensitivity and improve bandwidth, they are less un-linear than transistors being high impedance devices that require matching from an output transformer. Thus they sound better when used well, especially when used single-ended or in pseudo triode mode by connecting the grids together. Pentodes and tetrodes are more efficient (give higher static power) and much cheaper than triodes, this price advantage is paid for in poorer linearity and therefore overall open loop power bandwidth and load stability, nature always gives with one hand and takes with the other!
Directly heated triodes on the other hand are highly linear amplifying devices, the directly heated triode is the original voltage amplifier, the first, only and still the best, it responds well to better circuits, components and materials, but they are less efficient and more expensive than pentodes and tetrodes. Thus they require efficient speakers, with power output being at a premium price."
There is more written about why Negative Feedback sounds poorer than amplifiers that eschew it.
https://d1b89e86-9572-4311-9f80-600...d/3e7c3b_fd90b2b629a145709f59efd9116c652c.pdf
Martin Colloms back when he was the measurements guru at Stereophile
https://www.stereophile.com/reference/70/index.html
Peter van Willenswaard an engineer also writing for Stereophile Tubes do Something Special https://www.stereophile.com/features/357/index.html
