That said, among ESLs, SoundLabs are among the easiest to drive and thus best-sounding with tubes IME. Again just in my experience (also over decades), tube bass tends to sound "richer" or "fuller" due to added distortion but not as "tight" as SS, but I have not heard a modern tube amp on modern SoundLabs. And I completely agree that the output transformer of most tube amps is one of the overlooked components that distinguish "good" from "great".
Does that mean that ss. is leaner than the source or for that matter identicial?. If ss is not identicallto rl music then it distorts real music. Does that mean some have a preference for the poorer leaner sold state sound?
If I can shed some light on this:
Not all solid state amps are the same and neither are all tube amps.
We must also keep in mind that the Sound Labs are not easy for solid state amps to drive. This is because they are 30 ohms in the bass. For this reason a tube amp of 140 watts can keep up with a solid state amp rated at 600, for the simple reason that a 600 watt amp is going to make about 150 watts into that load.
Now regarding the 'sound' of amplifiers its really all about the distortion that amps make or do not. It is completely false to say that 'solid state amps are more neutral' when so many of them exhibit brightness and harshness as a coloration. This coloration is caused by distortion.
Now it can be solved if you have enough feedback, but to have enough feedback (in excess of 35dB) you have to jump through several hoops. First the amp has to have a phase margin that allows for the feedback without oscillation. The second thing you need is enough Gain Bandwidth Product; which is similar to gas in a car- when you run out of gas the car stops, when you run out of GBP there's no more feedback.
Because a good 99% of all solid state amps ever made have insufficient GBP they also have insufficient feedback. At 100Hz they have plenty and so measure well and play bass well. But at 1KHz and 10KHz the same amp into the same load has higher distortion. This is because feedback is decreasing with feedback due to insufficient GBP.
We've been hearing this problem for a long time.
IOW a desirable distortion characteristic is consistent distortion at all frequencies. The other desirable characteristic is that the lower orders (2nd and 3rd) are in sufficient amounts so that they can mask the higher orders. This allows the amplifier to have a smoother sound.
Now you can do this with zero feedback but you might get a fair amount of distortion. I'll make a further distinction here- there is a difference between a quadratic non-linearity and a cubic non-linearity. The former generates a 2nd order as the primary distortion product (SETs are a good example) while the latter makes a 3rd (at a much lower level). Of the two, the cubic is preferable (IMO/IME) because as the order of the harmonics is increased they fall off at a faster rate. This means that such an amp will sound smoother as it lacks the higher orders and the 3rd (which is treated by the ear the same way as it treats the 2nd) is masking them.
That is why we make zero feedback fully differential amps with no output transformer- we can get lower distortion with greater neutrality.
In a nutshell if you can identify either the 'tube' sound or the 'solid state' sound either way you're talking about distortion. The trick is to make the distortion inaudible by masking it with a lower order to which the ear is insensitive- and to that end keep the distortion otherwise as low as possible.
You can make a solid state amp sound nice and smooth- all you have to do is run enough feedback so it has the ability to clean up the distortion caused by the application of feedback itself (which happens thru a process called 'bifurcation'). You need at least 35dB to do that and frankly most designers don't bother and for many years (decades) the semiconductors needed to do that really didn't exist. They do now and have been around for a while but you still have to have the designer to recognize that and have the will to do something about it. One neat solution is to use a class D circuit and give it so much feedback that it exceeds its own phase margin and goes into oscillation. You then use the oscillation as the switching frequency. This type of class D amplifier is called 'self oscillating' and is capable of having enough feedback to get the job done. If you've been hearing about a class D project we've been working on its based on this principle. If the amp has the right distortion signature (and what we're finding is the signature is more important than the actual
amount of distortion) then the amp can be quite neutral, smooth and detailed- literally the best of both worlds.