Tube in Preamp or Power amp?

I doubt you will get a definitive answer to your question, but you do need to maintain system synergy.

For a tube amp, the speaker's impedance curve should not have substantial dips below the nominal impedance.

For a tube preamp, proper impedance matching to the SS amp must be observed.

If you are going to try a tube component, just make sure it will work properly with the rest of your system.

They do have a highish output impedance so make sure your amp's input impedance is a higher value than the pre's output.

Just being higher is too vague, as it should be at least 10 times higher at all frequencies.