I always liked the sound of that intro, figured it was a craqnked tube amp with an overdrive pedal though, it was recorded before the Fuzz Face existed I think, or any other fuzz boxes.
That was the 60's, effects were limited to wah, fuzz face and echoplex for the most part and people were doing all sorts of crazy things to get the sound they wanted. Lots of experimentation going on...
I think it was Ray Davies of the Kinks that admitted he had slashed the speaker in a Vox AC 30 with a razor blade to get more distortion and almost electrocuted himself.
The guitar played for the Guess Who supposedly had an amp designed to get the sound on American Woman, no amp or pedal he could find would do what he wanted so he had an amp built. Again before the Fuzz Face I think.
The Marshall Bluesbreaker was designed for Eric Clapton when he played for John Mayall's Bluesbreakers, basically the same story, he wanted an amp that would do what he wanted and couldn't get it with the standard Vox/Fender/Marshall choices at the time, pretty much the only amps available back then, with a few unheard of brands like Supro floating around here and there. (you suck by the way

And your damn old Supro too...

just kidding of course, I'm just jealous.)
That's how Frippertronics came about, Robert Fripp tinkering and trying to get more, various and diverse sounds out of a guitar and amp.
I wish that circuit had been availlable as a pedal, it would have probably been a major player...
Jimmy Page and David Gilmour also both used small Fenders in the studio a lot, Page would also mic the back of the cabinet as well as the traditional front mic, and if the stuff I read years ago was accurate he also used a Supro, ran the guitar straight into the board for a couple of solos, whatever he could come up with. I read very recently in an interview concering the Zep reunion he was using an overdrive pedal, wah (probably Vox from the looks of it) and an Echoplex, that's all he could get back then.
Same for early Gilmour, that's all they had and they had to make the best of it or have amps built/modified. Later on of course Gilmour turned into an effects nut, but his basic sound is still three 100 watt Hiwatt heads, and for studio still small Fenders mostly.
I'd like to know what other people did in the way of amp modifications to get the sound they did in the 60's with nothing but guitar, amp, overdrive, echoplex and wah available...slashing speakers, chaining amps together, onboard overdrive or fuzz...Johnny Winter was using 3 Fender Super Reverbs wired together, Ted Nugent a wall of Twin Reverbs, Hendrix 3 Marshalls...