Good stuff, as usual.
Its good to know detailed impedance measurements when available, but in lieu of that a good rule of thumb seems to be to use an amp with 60Kohm or higher input impedance to be safe with most tube amps. Slightly higher than the 10:1 minimum (as applied to worst case measurement at particular frequency) usually cited is a good and safe insurance policy.
FWIW, you will find most amps that explicitly mention or support use with tube pre-amps tend to have at least 50-60Kohm input impedance spec, often 100kohm unbalanced and twice that even balanced, which is usually preferred for the best and most consistent results.
Its good to know detailed impedance measurements when available, but in lieu of that a good rule of thumb seems to be to use an amp with 60Kohm or higher input impedance to be safe with most tube amps. Slightly higher than the 10:1 minimum (as applied to worst case measurement at particular frequency) usually cited is a good and safe insurance policy.
FWIW, you will find most amps that explicitly mention or support use with tube pre-amps tend to have at least 50-60Kohm input impedance spec, often 100kohm unbalanced and twice that even balanced, which is usually preferred for the best and most consistent results.