Expand the above quote to see my answers.
Thanks I have replaced ALL of the cables, taken the unit in and out of the rack, gone back to the XL...
A lot of good answers there. Having a silent rig when you roll of the volume is really good. Let’s dig into the items that you should look at to ensure they are not causing problems.
You indicate that you are using “XLR and humbuster where required”. I take it that you mean that you have tried both XLR and Humbusters on Output1? The SLA power amps I just looked at have an XLR and TRS input for each channel. Since the Axe 1/4” Ouptut 1 is balanced (TRS) and the SLA 1/4” input is balanced (TRS), you would use a TRS Cable if connecting via the 1/4” jack. But using XLR is equivalent, so if that did not help...
You indicate that the power amp and Axe are in a rack. The rack chassis is probably adding noise. If the screws contact any metal on the chassis, the screw will bond that chassis ground to the rack rails. If a second unit in the rack also gets its ground bonded to the rack rails, you have a ground loop. That will cause noise. Get you hands on some humfrees or similar type of rack isolation tabs. They are really cheap.
https://www.guitarcenter.com/Humfre...MIzdXJvuDB4gIVDFqGCh3kHA8qEAQYAiABEgJf7fD_BwE
The noise that can come in on your power is something a lot of guitarist do not appreciate IMO. If you care about noise, you need to invest in a quality power management solution like the Equi=Tech Son Of Q or a Furman Prestige 1800. I use both the Furman P-1800 AR and PF-R. It makes a difference, but it is not cheap. The Son of Q is supposed to be the absolute shit.
Ultimately, the 100% proper way to tame noise in rigs is by using a bonafide star grounding scheme. That is a pretty technical endeavor though. Everything else is essentially a band aid.
Outside of those thoughts, the noise you are experiencing almost certainly includes some inducted noise being picked up by your guitar pickups. There is also the factor that the Axe III seems to be more sensitive. My gut says it is probably a mix of greater sensitivity of the Axe and the noise gate changes along with some other noise sources (e.g. power, rack ground loop, EMI) that pushed this over the edge for you.
I do not envy you. My rig is finally whisper silent, but it took a lot of learning, time, and work to get there. Dig in learn about noise in sound systems. Experiment with different cables. It really is a multi-layered puzzle and you have to determine how far you are willing to go.
EDIT: I made heavy use of the meter views to see what my inputs and outputs were doing that helped me to isolate different noise sources. Also read the section of the manual on setting levels. There is also something in the manual about lowering the noise floor on the Axe, you should play with those things too.