Negative feedback, voltage and output impedance

As a general rule, higher amounts of negative feedback will lower the output impedance. If you want a technical explanation of this, your best bet would probably be to head over to the Tube Asylum and ask there. Sean
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Output impedance in tube amp solely depends on output transformer except cases with OTL where the complicated circuitry applied for the output stages to decrease an output impedance.
Negative feedbeck realy doesn't care for tubes or SS and neccessary to increase the freequency bandwidth and decrease harmonic distortions.

Wasn't there a relatively recent review of a tube amp in Stereophile that had user adjustable negative feedback? If i can remember correctly, this not only affected the output impedance but also the bandwidth / linearity of the entire design. Was this the ASL amp??? Sean
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Sean,

The only amp I recall with user adjustable feedback is the Mesa Baron. I believe it is adjustable between 0 and 8db in 4 increments.

This is an amp I am considering purchasing, btw. It is a very flexible (can also switch between all triode and all pentode in 1/3 increments) dual mono amp and may be what I need in order to determine what works best with my ESLs (other than OTLs of course).

Kevin Hayes has also offered to make some mods to a PA100 for me which is another option.

Thanks.

Mike