Could be that you're sending the wrong level signal to the X-Amp.

Try changing the output level for the Fireface in Settings.
You have three settings for both input/output levels. Not sure which one the X-Amp is looking for.

To cover all bases - check your cables, and also for ground loops.

I've tried adjusting the output level on the fireface and the X-amp.

The output level on the fireface has no effect on the hum, but the output on the X-Amp will of course lower the hum, still you need it at least half way up to get a decent signal into your amp.

I haven't tried other cable because I don't have them :)

I'll try it in another house tomorrow, wish me luck!

You might want to invest in a cable tester at some point. Your cable could be wired incorrectly even if it is a factory cable. I'm not familiar with the X-Amp but it is unlikely a FF problem unless the X-Amp is looking for an unbalanced signal.

Hmm i'm not sure if the output is supposed to be unbalanced, the Input of the X-Amp says "Balanced Input 600 OHM".

And the manual says connect with a balanced cable.

Hey Jack, if he were going to check the connections of the TRS to XLR cable using a multimeter, would he meter it like this?

Tip to pin 2
Ring to pin 3
Sleeve to pin 1

Yes. That is the US standard. It should also be listed in his Radial manual somewhere and often is imprinted on the case too.

Sounds like the common wire has come off, either your patch cable of the unit. How is the ground lift cap doing? Are there any caps to common in the audio path? Is that passive box? If so they likely used this... as basic circuit.
.jensentransformers.com/as/as092.pdf

I remember him mentioning before that he is using it with a 120v amp (USA) while running the reamp box and everything else using 240v power. Related?

Is the neutral common?

Here's a picture.

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A JRC 4558 for an opamp? Now what would a lo-fi opamp be doing in a piece of hi-fi gear lol? I suppose that would be to add 'color'?

My guess is that the big balloon shaped tantalum cap next to the ground loop switch is the ground lift cap. Looks like C9.

lol- theres nothing wrong with that chip for this application....

Thanks for the pics!

Well we now know its not passive.....

I would think he would not be using a tantalum cap for the ground lift circuit.
The one closest to the XLR connector would be my guess, maybe the poly brown guy.

From what I see in the pictures nothing is obviously damaged.

Is it true that you 220V 50 Hz system for everything but this unit?

No the unit is EU but the amp is 120v.

How are you converting the 220 to 50 Hz to 120V 60hz? and how is the 120V neutral connected? How are the grounds connected?

Have you tried moving the power supply for the X-AMP to the 120V 60Hz source? make sure your walwart can do this!

Yeah I've got a big converter for the 120V amp, I've plugged the stuff into the same outlet source, which didn't make a difference.

On other news, I tried using output B on the X-Amp and engaged the side ground lift of the X-amp, wouldn't you know it, all the hum was gone.
Still when I plug into Out A the hum comes back, I'll live with this for now.

Output B is transformer Isolated, you have grounding issue.

Think a power-conditioner is something I need ?

Would not hurt, but I think you 120V neutral is tied properly into 220V grounding system correctly. The way is to uses isolation XFMR like output B.