Well I'd be using the GX-100 in front of a properly grounded tube amp.
Don't know much about the innards of the Boss and its grounding scheme, but the issue with a typical grounding-induced hum/buzz grounding was most likely caused by a ground loop.
I had everything connected via signal cables AND switching cables between:
- amp (signal, and channel switching by the GX 100)
- GX-100 (signal, and switching of amp and relay bypass looper)
- relay bypass looper (signal, and switching by the GX 100)
...AND all this powering off of only one power supply. Well I know, this may not the "official" way to do it, but since Ihad good experiences with daisy chaning pedals with the small Boss supply, I thought, let's just try this.
As expected I had the hum/buzz, which I solved by putting a Joyo ZGP in the 9V line feeding the relay pypass looper. Dead quiet! Also works with the big Boss supply, but this setup still has the noise component in the 500...800Hz region, which I also have only putting the GX-100 for itself in front of the amp.
Well, while getting back in touch now with the GX 100 after a few months, I rapidly start to get sick of browsing through menus and parameters AGAIN. Although I get along quite well with it, nothing can beat simple stomboxes for intuitivity, at least for me. So I might get rid of my GX-100 soon anyway... maybe ...
Ok, enough for the hijacking (well, which I started ... kinda...).