I asked a similar question on Dynaco Doctor but feel like I need more input...
I'm using the VTA driver board and no longer require the stock 10K pots on the ST70 for biasing purposes. In their place, I would like to wire up some high-quality pots (or stepped attenuators) for use in my negative feedback network.
My thinking is as follows, and needs double-checking:
Each channel would have a pot in series with a resistor of the recommended lowest resistance (in other words, the maximum NFB before the amp became unstable). Each pot would be of such a value that in the middle position each NFB channel would have a series resistance of 3.8Kohm (my fixed resistance now, running from the 4-ohm tap). For example, a 2.7Kohm resistor in series with a 2Kohm pot. This would yield a variable range of +/-1Kohm.
I've seen amps with variable feedback controls so you can tune NFB to suit your listening tastes, but have little idea how complicated a procedure it would be to put similar controls in an ST70. I could use some feedback, if you pardon the pun.
Best,
Brinkman