Thank God I found this thread! I've had the softstep for about a year and love it. Last summer I modified a Les Paul copy with a custom wiring harness with Jimmy Page style mods and SD Pearly Gates pickups (Humbuckers). The JP mod to the LP uses four push pull pots to allow you to split either pickup, and to have them be in series or parallel, and out of phase. That adds a LOT of wire in the guitar. Well, when I got it all put together I had a HORRIFIC hum - that I hadn't had with any of my other guitars (a coupe starts, and a tele). I ended up shipping the guitar to the guy who made the harness for me. Got it back recently and I still had the bad hum. I checked online for solutions and checked everything on the guitar that I could and couldn't find any issues - everything is ground (verified with a VOM). So I started messing with my audio rack. Here is a picture of my setup:
I found that the problem was coming from the SoftStep. If I picked up the softstep and got it near the guitar the noise greatly increased. I have a number of USB devices (Line6 Pod HD500, Zoom B9.1ut, Behringer X1204USB mixer, SoftStep) - they were all plugged into a powered USB hub. I tried hooking up the Softstep in a number of different ways - with a shielded USB with ferrite chokes (along with all the other USB cables I have on hand), running it through the powered hub, connecting it direct to the Macbook Pro, having it be the only device plugged into the MacBook, running the Macbook on battery (not plugged into AC), etc. No matter what I did as soon as the SoftStep was powered up I had the hum.
So I went to the McMillen site to contact support and found the forum here and searched on it and found this thread. So I went in and turned off the EL lighting - and instantly the hum was gone. So then I plugged in some of my other guitars and discovered that I had the hum with those as well if the lighting was on - it was just much less so I didn't really notice it. I was surprised because those guitars have single coils versus the dual hum buckers on the LP. The difference being the Pearly Gates are higher output coils and the LP has a LOT more wiring in it due to the Jimmy Page mods.
So the lighting will be staying off on my SoftStep. I would prefer to have the lighting on because it definitely makes it easier to use and looks great - but I can't live with the hum. Thanks for the solution to my problem - this has been a 6 month ordeal thinking it was a problem with this guitar and I was at my wits end with it.