I was looking at my new Gered Mankowitz book; many great shots from 65,66. There are pics on stage in USA '65. He was photographing from the side of the stage so you can see behind the amps. They are using Fender amps.
There is on pic of them still setting up, and it looks like the cord that someone is plugging into the back of Bill's amp is a low impedence jack. Well it LOOKS like the connceting cord from speaker to amp is lo impedence;
then a 1/4 " jack that is also in the back of the head runs off picture but towards the back; did they have direct boxes? Would that even make sense? I thought at first it was Keith's amp because it is small to be a bass amp, but one can see Keith's amp closer to Charlie. On the next page it's a bit clearer, although Charlie sits in cable spaghetti. On Brian's amp one can see that there is what looks to be a lo impedence jack coming out of the speaker cab.
I'm just wondering why they are using lo impedence jacks on the guitar amps.
Was it a crude/ingenious way to deal with noise from all the wires and cables, plus lights?

Circa 1965 Bill usually used a Vox AC100 with 2 50 watt foundation bass speaker cabs, one cab on either side of stage. At the time Vox used 3 or 4 pin XLR connectors for both the power lead(yikes!) and the speaker leads.

I assume this is what you are seeing.