The logic behind NFB is how much negative feedback you put in place, it is measured in dB so you usually hear statements like "this amp has 20dB of negative feedback".
How to measure it? Take the open loop gain in dB (the gain of the amp without the loop in place) and subtract the closed loop gain in dB (the gain of the amp with the loop in place), this gives you how many dBs of negative feedback has your amp.
Amp distortion (below clipping) is reduced by the same amount of negative feedback, although the harmonic spectrum become worst, while the damping factor is increases by the same amount. (i.e. 3dB of negative feedback means half distortion and double damping factor)
If the amp has an "almost" infinite open loop gain like usual opamps, the amount of negative feedback can be easly calculated from the ratio of feedback and shunt resistor. Tube power amps have a relatively low open loop gain so calculation isn't easy but you can find all the formulas on the Aiken site.
I usually prefer the PC way, so I just simulate the amps via Spice to take open and closed loop measures, you can do it directly on the real amp by inputting a 1volt (or less) sine signal and measuring the output voltage at the speaker with and without the loop in place. Then tweak the resistors until you get the amount of negative feedback you desire.
The 100w amp should be between 5dB and 6dB ->
http://www.m-elli.it/dumble_50w-100w_feedback.pdf
teo
