NFB as a theoretical concept is not bad for audio signal reproduction, not bad at all.
For example, the reason a triode with good curves is linear, is because of NFB inside the tube.
The electron stream traveling in vacuum and different electric fields (such as the field around the control grid) has a feedback loop in it. This is what linearizes a triode.
The strenght of this NFB inside a triode is dependent on the geometry of the tube; how the electrodes (cathode/filament, control grid, anode) are placed in relation to each other.
There are triodes with "tetrode like curves" that don't have the screen grid. This is because the distance between the cathode and anode is so great that the feedback effect is very weak. (Very very high rp)
There are even triodes with pentode like curves (although a weak pentode); they even have beam plates! They are called beam triodes. They are usually for some extremely high voltage duties. The distance between the electrodes is so great, the tube can withstand something like 10kV of voltage. Good for some radar and large screen CRT applications.
For audio use triodes have a lot of NFB inside them. This is not bad, because what makes NFB bad is WHAT IS INSIDE THE LOOP. In a triode, there is vacuum.
Put a resistor inside a NFB loop. Not bad. (For example a CCS)
Put a capacitor inside a NFB loop. Not bad (if decent film cap).
Put a resistor and a capacitor inside NFB loop; not bad at all. (Say, the gyrator.)
Put a complex system of voltage and/or current gain stages inside a NFB loop, and you've got big big problems. This is the SS SOUND.
A regulator is a 100% degenerated NFB loop. The loop is local, very short, and doesn't contain any significant amount of reactive elements. No SS nastiness or problems.
The Little Dot's 'regulator' in the PSU (it's really a voltage setter, since the feedback loop is only (mainly) affecting the PSU ripple, not the audio side of the circuit; it sets the voltage, it doesn't REGULATE the B+) has a 100% degenerated NFB loop. Does it bring SS nastiness into the sound?
SS is not bad inherently. If you put SS inside a local, small and simple NFB loop, it's just as un-nasty as a triode with it's local, small and simple NFB loop.
This is a rudimentary explanation of the subject, but just to illuminate a bit about these things.
A tetrode or a pentode don't have triode curves because the screen grid breaks the NFB loop between the plate and the cathode. It breaks it by having an independent electric field in the path of the electron stream. If you want to have your pentode behave more like an ideal pentode (so, more like a MOSFET in essence), you regulate the screen supply, making it very sturdy. This makes the screen grid electric field more independent of the anode conditions, and breaks the feedback loop in the electron stream more, thereby making the curves more horizontal and flat.
If you connect the screen to the plate, you no longer break the NFB loop, therefore you have more linear curves. After you do that, the linearity of the system is again dependent on the inner geometrics of the tube. Most tetrodes, beam tetrodes or pentodes are not designed with triode operation in mind, so they have rather poor geometry for triode operation linearity. There are a notable number of exceptions, such as 4P1L.
4P1L was actually designed by nazi scientists. It was probably meant for a very wide variety of applications, and that is why it was designed with geometry suitable for triode operation as well.
Most pentodes were not. Linear triodes were around, so why bother? The nazis were operating under very severe shortages, and couldn't afford to make that many different types (each needing their own factories etc.) Same goes for the soviets.
People think because SS doing voltage amplification necessitates a LOT of NFB to make it linear, thereby causing A LOT of SS nastiness, that all SS components by nature produce this nastiness in all circuits, no matter what they are used for.
A general rule; if the SS component has 100% degeneration, meaning it exhibits no voltage gain, it WILL NOT PRODUCE ANY NASTINESS. At all. No more than your 1930's triodes. Actually, less than that if the 1930's triode is exhibiting voltage gain, it will produce some harmonics. The 100% degenerated SS component will not produce any harmonics, because 100% degeneration means just that.
If you have 97% degeneration, you will have harmonics.
In short, it's simply idiotic to compare SS with voltage gain to SS with no voltage gain. Any articles discussing the first and reaching a conclusion IT'S BAD FOLKS is not applicable to source followers or 100% degenerated regulators and such. They are different things.
A CCS is not exhibiting any voltage gain to the circuit. There is gain inside the loop, but it's 100% degenerated. This means 100% of the gain is dissipated in the loop, in audio frequencies. Sometimes there is gain with radio frequencies (oscillation) and that needs to be dealt with.
A CCS will not contribute any signature to the audio, other than by correctly organizing the circuit for example balancing a triode pair in a LTP, or sinking grid current from a power tube grid.