why so important cathode-follower if you don't mind to reveal?
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If you are trying to drive an amp with a low input impedance, and you want tubes then you will need one that has a cathode-follower or Mu-follower output.
However the amp has a balanced input so you might consider tube preamps that are balanced as well. I prefer tube preamps with a direct-coupled output in this case, as they play bass much better with greater extension.
I think you are going to find CF's in most of the tube preamps you go looking
for. It's not the spec I would go looking for.
fwiw, I have 2 Pass amps (30.5 and 100.5's) and I drive them with Conrad
Johnson tube preamps (ART and ACT 2.2) and neither of them have CF's. Their
output impedance "averages" 500 ohms and is not a problem for me. My Pass
amps are rated at 20K or 30k (rca or balanced).
Whatever the input impedance is of your Pass is, just look for a preamp that is
at least 1/10 of that and hopefully a little more because the output impedance
does vary at different frequencies. That spec is just an average.
If you have a direct-coupled output on the preamp the output impedance will not vary with frequency (with most tube preamps its usually the bass where this is an issue).
If you have a 10:1 ratio at 1000Hz but only 5:1 at 20Hz, you will hear that as a loss of bass impact. This is because there will be a slight loss of output voltage in the lower frequencies.
To answer your original question...try Mystere CA11 or CA21 preamps. The CA11 uses a SRPP (Series Regulated Push-Pull) circuit for high gain, low distortion and very low output impedance by using a variation of the White Cathode Follower. The circuit uses no negative feedback for more natural tonality.