preamp inverts polarity

geoff, read their posts again...they said “ decompress” air not compress. Either way, you’re correct about how vacuum is obtained. 
I’m still stuck on that single ended amp remark...😁

Kalali,

It’s pretty simple. The bias level in a single ended amp defines how negative a signal it can pull it down. If the source drives it below that bias level, the gain device goes more non-linear and eventually shuts off completely. It’s the electrical equivalent of trying to pull a vacuum in air. Push-pull amps don’t do that unless they’re deliberately mis-balanced. Instead, they transition to class AB and one half of the push-pull does all the work. That's why SET amps have such devoted followers. The way a triode distorts as it runs out of bias is very organic and similar to how air behaves.

@kalali

Just to be clear here, Push-Pull amps can be class A and also all triode. The 'air is single-ended' thing is a bit sketchy and also a bit of a red herring, since air has nothing to do with how amplifiers operate.

The transmission characteristics of air certainly don't resemble electrically balanced operation. And no doubt there's more to the character of a triode than how it distorts in a SET. Its undeniable that many fans of SETs like them because they distort in a way somewhat similar to how air does. 

Even if what you say is true, which I actually doubt, it would not change the fact that at least 50% of CDs are mastered in reverse Polarity. SETs don’t magically counteract Polarity. There is no balm in Gilead. Not to mention the fact a lot of people don’t like the sound of SETs.