All devices should follow the rule of low Ro and high Rin.

10 to 1 good enough then? Or is some overkill still a good idea?

10:1 is only 600 to 6k.

Overkill is better. Tube amps have typically 100k input Z and
CF output from preamps = 600 ohms.
SS pres have opamps, maybe Ro = 20 ohms, but usually
they place a 470 ohm R in series to the output terminal which
stops the opamp fusing itself dead if a shorted output occurs.



Your power amp should be configured to make full power at a volt
input including NFB.
The soundcard can make up to a volt output, no?

Yes that is what I strive for. I have gone to direct connection from
quality soundcard because a preamp is simply not necessary.

The Lynx is amazing. +/-100 ohm Zout. Yet apparently designed to
function perfectly driving 1V into a 600 ohm load or higher.

If that is possible, then its ok to drive low impedances of 600 ohms.
It is very easy to make a robust output stage after an opamp
which will have low thd and able to put maybe 5 vrms into 100 ohms.
Its only 50 mA rms of current, and easy from a pair of class AB bjt
transistors
after an opamp and included in the loop of NFB.



Patrick Turner.