They should do.
An ECC82 with a few Megohm from anode to control grid operates greatly at even less than 6V anode potential as a mic head amplifier. See Pleiades V0.
Good candidates of Nuvistors should be high gm, vari μ, with screen or suppressor grid connected to anode. Preferable a multi grid electron tube but a triode should work too.
Underheating can be done with beneficial results. For example the AKG C12A condenser microphone has its Nuvistor head amp supplied by 5V heaters instead of 6.3V.
With Pleiades bias, low anode voltage the heaters should be optimum at less than 4-5V. (A high temperature increases electron cloud negative bias). Less temperature should further minimize secondary electron emission effects from all electrodes
These low noise circuits are suitable for moving coil, ribbon microphones, see Pleiades V6.
Nunvistor candidates can for example be 7586, 6DS4, 2DS4. See nearby euroelectron post.
How would they sound powered by USB?
Can a complete electron tube mic preamp with input transformer and A to D converter be made?
References:
The Pleiades bias - euroelectron
Pleiades V6 schematic
On preserving transconductance of electron tubes at anode potential as low as 3V - euroelectron
http://www.mif.pg.gda.pl/homepages/frank/other/RCA/RCA_Nuvistor_Tubes.pdf
https://frank.pocnet.net/sheets/049/6/6DS4.pdf
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