Helix LT 1/4" Output strange noise

[ARICHS]

I've been using a Helix LT for a few years now and and recently bought a poweramp (Warwick Gnome mini bass head - Not technically a guitar poweramp but does the exact same thing)and 2x12 to play live.

The setup sounds great, but I've noticed a squealing/humming sound when the guitar signal is silent or the amp is on but not being used. I have previously been using the Helix into the Effects Return of an amp (but I don't currently have access to this) and I don't recall this noise there. (Although maybe it was because I always wear ear plugs at practice).

 

The hum sounds almost like the standing 'hiss' from a loud amp with overdrive but no signal, but plus a tinnitus-esque electronic squeal. It persists when even the Helix is switched off, so I also don't think its a software issue on the Helix.

 

I've done some experimentation to isolate the Helix:

 

• When my signal is Guitar>poweramp, or Guitar>Normal pedal>Poweramp it sounds fine.
• Swapping all the 1/4" cables used, as well as the power supply to the poweramp and Helix (direct to wall vs extension lead), and unplugging everything except the 1/4" output on Helix (including the guitar).
• I've also tried it in every other 1/4" plug on the Helix, whether its an input or output jack.

• Helix>Headphones and Helix>HiFi via Aux (via phones jack and both mono outs) have no squeal.
• Using headphones from the poweramp also has the squeal.

 

The issue has got to be somewhere between the Helix output and the Poweramp input, but the fact that the sound persists when the Helix is off is really weird.

 

My full signal chain in full is:

Guitar > Instrument cable > Helix Input > Helix 1/4" Output > Instrument cable > Gnome Input > Speaker cable > Cab Input.

 

In the Helix settings I've switched between Line ad Instrument levels and various other settings, as well as Ground Lift (even though I'm not using XLR) on both the Helix and the poweramp.

 

After some light searching I found a post on the Line 6 forum that details something similar, but he was "cured" by using an "impedance balanced TRS cable" I'm not even going to begin to pretend I know what that means but I'll try and read up.

Alternatively, I have wondered if an XLR-1/4" cable would help, but don't currently have access to one.

 

Can anyone offer potential insight to the cause of my issue here? It isn't loud, and it can't be heard over playing, but I know it shouldn't be there, and it does increase in volume in line with the poweramp output so I expect at band volumes it may be noticeable.

 

I'm running out of ideas here. I'm not sure where the issue is, since helix output to any other speaker is fine, and poweramp input without the helix is fine too.

 

Thanks for reading!