The Fet Valve 400R's AC input impedance is approximately 500k ohms. Its DC input resistance is approximately 1 Meg ohms.
The SET 400's (AC) input impedance is approximately 60k ohms -typically, as it depends in the beta of the input transistor. Its DC input resistance is approximately 1 Meg ohms.
All the Fet Valve amps have the same Fet Valve input circuit, and all the SET amps have the same SET input circuit.
Any preamp worth its salt should be able to drive a 10k ohm load without issue. The SET's 60k input impedance is much easier to drive than that (like 6 times easier). The Fet Valve amp is an even easier load to drive.
Dan
