Speaker Hum, no the usual problem tho! read..

[Steve Zorilow]

@DJKeyWee What is that line tester you posted?

That's a common line tester we use and find in most store in here. Places like Home Depot and the likes should have it. It's something like 15$

If you have no audio connected then the inputs are floating and will amplify mains 60Hz for US and 50Hz for Uk,

Not always true. Even without anything plugged in the input jack AND with volume set to 0 (minimum), if the outlet is badly wired (Hot wire in place of Netral, no ground and so forth) you may hear some nasty hum on some Amplifier, Amplified Speakers brands, depending how they're internally designed some are more "sensitive"to that.

If you're using the same gear on a properly wired outlet and it fixed the problem... then you have a good idea that you got a bad outlet before.

The tester I posted here will let you know what's wrong... or give some pointer assuming it's an outlet related problem.

Even if you use balanced wire, you may solve or not the problem. In some case, you want to use a simple Balanced wire/adapter but with the Pin 1 (shield) lifted. Isolation Transformer (Passive DI) are always good solution if they also have a ground lifting switch.

I did touring all 'round the world as PA engineer and damn, it's sometime a nightmare to solve them... specially when you're using lots of KVA power distributions.

There's a load of trick we used, but some are too technical, or not "Kasher" so I don't suggest them... even if they works

the gain involved is huge and any slight difference in a transistor pairs matching will give you noise.

HIgh likely a CMRR problem here, more often leaky input Capacitor (high esr and so forth) or faulty/leaky input op-amp.