How come I didn't get earth loop hum?

I've been playing for yonks, but never before bothered to run two amps in stereo....

Anyway, I was in the rehearsal studio and just fancied giving it a go. I didn't use an earth loop hum eliminator, and I figured that there'd be major hum, especially given the state of the wiring in that place (4-way extension leads screwed to the wall, all linked to the one mains outlet). But there was no more hum than is usual for me.

I took one lead out of my mono DD3 to the practice place's Marshall Valvestate, the other lead out of my analogue Ibanez chorus to the place's fender Roc-Pro.

So: was it luck, the amps, the fact that the leads were coming from separate pedals, or what? For the record, the dual amp setup sounded much more meaty...