Will doing this blow my speakers?

Use a gray 3 prong to 2 prong cheater plug on your power cords to lift the ground. If you have a AC ground loop you can use the cheater plug to find it.

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.

I had an AC hum problem in my stereo system and it ended up being the cable feed from my local TV cable provider. The fix was a filter where the cable hooked up to the digital cable box. Just thought I would mention this to you as I never considered this as a possible source.

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