This is normal and you will find that any class-A preamp does this (unless there is a relay circuit to delay the switching of the output - normally found in active speakers).

The noise is caused by all of the components suddenly being charged by a DC so there is no way to prevent it.

With any recording set up, you must turn devices on in the order of the signal chain -always amp/monitors last.

Make sure the Voicemaster Pro is the first thing you turn on and the last thing you turn on in your set up.