??slight hum from the speakers ?

Usher mini-dancer 2 speakers
Emotiva XPA-1 monoblocks, Emotiva ERC-2 CD player
Mystere ca21 pre-amp

Just received the Mystere pre-amp & noticed a slight hum when nothing is playing, compared to dead silence with my other Emotiva pre-amp UPS1

Is it to be expected with a valve pre-amp??
It's possible your interconnect cables are running too close to the power transformer on your tube preamp.
You might have a ground loop, tube going bad, or a bad connection on the sockets or all 3.

You can use a cheater plug to lift the ground but I would highly not recommend it.
A slight hum coming from a tubed pre-amp could be from poor contact between the tube pins and the socket, or noisy tubes. Try removing and replacing tubes, tensioning the contacts in the tube sockets if you can, or having a spare pair and change them out and see if there is a difference.

And perhaps Hifigeek is correct and it is nothing much more than cable and IC dressing. I have an amp which is very sensitive to PC placement. Try to keep PC's and IC's not only away from transformers but running parallel with each other (crossed should be OK).
will try your suggestions & let you know how I go
I must admit you can hear the crackling (faint) as the pre-amp warms up so changeing or replacing the tubes might be the way to go