Capacitor

Get a 3uF capacitor. If I were you I’d go with a higher rated voltage. I cannot tell if you’ll roll off the bass by going 1uF without a circuit diagram, but I suspect you will.

Your post suggests these caps are on the output of your preamp, please confirm.  We need to know to output impedance of your preamp. Please share that. Also please share the input impedance of the amp you are using. With this info I can help you decide. 
I think the Milflex are good, but not great.  The VH Audio Odams sound better in every sonic way and are much smaller making for easier fit. 
Let us know. 

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