You will not hurt your speakers doing this.
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Solid state amps are ok to do (no damage to the speakers or amp)
Tube amps I am not certain about. (I only know for certain do NOT turn on/off a tube amp with no load.. ie: no speakers, as this can blow up the amp)
And yeah, the way to find hum is to do one part at a time.
I would first disconnect all sources from per, then see if pre/amp combo hum.
If so, try just amp.
Usually a hum is a ground loop in the nest of wires... in cluding power cords and grounds.. or a bad interconnect...
The most common hum is an ungrounded TT or a poorly grounded turntable.
One way an amp can damage a speaker is by supplying DC current, either by internal damage or a faulty soft-start circuit (the "thud" you hear when powering up). This, if it happens, is independent of downstream source components. If your amp is okay, I agree, you can safely power it up with no source components.
I don't think you need to be reminded to change connections only while every component is unplugged. Don't place faith in the power switch alone.
Thanks all for your thoughtful responses. I should have said in my original post that the hum completely disappears when I lift the ground on my amps going to the wall socket.
But, I want to try the amps (monoblocs) with my new Equitech 1.5Q and Equitech advises against lifting the ground when using their device.
Plugged into wall - ground lifted ... no hum
Plugged into wall - not lifted ... hum
Plugged into Equitech - not lifted ...hum
Plugged into Equitech - lifted ... not tried.
I'll give removing the preamp cables a go.
Best regards, Jeff