Capacitor

Where is the cap? What is the circuit? In the absence of solid understanding the safest course is to use only same value caps. They can vary by a bit but in percentage terms, okay? The values you are talking while small in absolute terms are large in percentage terms.   

I have swapped caps out many times, and always with great results. But always paying close attention and keeping values identical or worst case within a few percent. Most parts are within a few percent tolerance anyway. Take this approach and you will avoid a lot of potential problems.

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