Probably Grateful Dead gear. They used Sunn double 15s as monitors regularly back then and helped set up the debacle. Looks they were pretty useful for keeping the crowd off the stage, too!
Before the show that's also what they said about paying Hell's Angels in beer to handle security...
I wonder what kind of drug influence that decision, and who's brainstorm it was??? (Not that they'd admit it.)
It was complicated. The Stones had used London Hell's Angels before at the Hyde Park show, but those guys were not really like the American Angels, more like guys making a fashion statement. But, they bought into the romantic mythology of the Angels and wanted the true California experience. So, the Dead were friends with some of them going back to the Acid Test days (When you put out a sign that says "The Merry Pranksters welcome the Hell's Angels!" wild things are sure to ensue and they did.) and talked to them about it. However, at that time of the concert, most of the Angels they knew were at a funeral and the Angels who showed up included a lot of guys who were unfamiliar with the band scene and the bands and were trying to prove themselves. In retrospect, it was one of those things where they should have known better, but neither band had had a serious problem in the past. While the Dead knew of the violence that they guys could bring, they had personal relationships that had kept things under control. Too bad none of those guys were there to keep a lid on the newbies.
The big screw up was that the Dead and Diggers arranged the show at first to be in Golden Gate Park and got all the permits. The only caveat was that absolutely secrecy had to be maintained until 24 hours before the show. But that self-aggrandizing, prancing, egotistical maniac Jagger (which, of course, is part of the charm that allows him to be such a compelling performer, if you are into the rooster on acid kind of thing) couldn't keep his mouth shut and held a press conference announcing the show two weeks out. If he had just kept quiet about the whole thing as per the original agreement, it could have been in the park and far less crazy and probably a great show. I pretty much lay this all on the Stones. The Dead had ongoing relationships they put on the line, with San Francisco, with the Angels and a number of other groups and they had nothing to gain by being reckless. The Stones, on the other hand, trashed the whole thing like a hotel room and then went back home to England.
I guess you can tell where my sympathies lie. Not with the devil.