Arny Krueger wrote:

What would stop one from doing state-variable filters with
tube op amps, patterned ofter the Philbrick modules of the
days of tubes?

Slew rate and bandwidth. Those Philbrick and HP tube op-amps
really did not have the bandwidth for anything approaching audio.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."