Capacitor

Message @dgarretson, the King of Capacitors for his input! Cheers,
Spencer

@grannyring
the output impedance is 1200 ohms and the input impedance of the amp is 470000 ohms

It is my impression/memory that 2uF is the minimum size for a preamp output capacitor.

1uF ~= 8 kOhms at 20 Hz. Calculate the ratio of this to the amplifier’s input impedance to see how much you would lose.

https://www.calculatorhut.com/physics/reactance.html

If your amp’s input impedance is really that high - 470k, then even a .22uf is fine! To assure it works with most any SS amp you may get in the future, then I would suggest any value from 1.5 uf - 2.2uf. It is best to keep the cap value as low as is reasonable as it helps sound quality. Bigger value caps, of the same brand, won’t sound quite as good as smaller values in that output position. Your preamp’s output impedance is not crazy high, but likely varies by frequency and volume. Depends on the circuit.
To assure no bass roll off with most any SS amp in the future, coupled with my comment on larger cap values, stick with 1.5 - 2.2 uf. I see no reason to use a higher value cap. I have built many tube preamps with output caps values of .47uf to 1.5uf with no issues at all feeding SS amps with input impedances of 47k and over. A 2.2uf value VH Audio Odam is my strong recommendation.  

@grannyring
The preamp is C1800 of canary audio and amps are grand reference from same canary audio for your reference