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Quote2. Why do the circuits need a negative supply on the source and why do that supply need to be buffered?

Because with autobias with source network tied to GND I can`t get stage gain, frequency response & top/botton clipping points same as for tube prototype.

I take it that you don't have to use negative supply if you are NOT trying to emulate some known amp. In fact in this JCM800 emulator only three first gain stages actually use additional negative biasing.

How about using a lower supply voltage in range of 48-100V?