I think it was Rudy Leiren, Eddie's old tech who talked about Eddie slaving the Bandmaster into other amps using the extension speaker out.
There is a 1985 Steve Rosen interview with the real Robin "Rudy" Leiren and the slaving theory before 1984 is destroyed by Rudy.
Rudy: We already had, uh…most of the amps. And for the first time in 1984, we were uh…we were bi-amping…the guitar. I was feeding, uh…signal from, uh…from a Marshall into, um…some H&H power amps. Every other year previous we had powered everything onstage with a Marshall for every cabinet. But you know…I…like…you know…as you know, one Marshall doesn’t necessarily sound like the next…
SR: Right…
Rudy: …Marshall. And so, Edward always used to complain when he used to walk up across the stage…or when he’d stand on the riser, at how bad the guitar sounded, you know like for Alex, you know, or f-…or on Michael’s side of the stage. And then last year with uh…with uh…(the) use of the power amps, everything sounded the same across the stage.
SR: Mmm…
Rudy: Everything was the same. Ever-…I… you know, like…In 1984, Edward did spend a lot more time, like, standing up on the riser…
SR: Yeah…
Rudy: …jammin’ with Alex.
SR: Right.
Rudy: Because it sounded good…
SR: Yeah…right…
Rudy: …up there. For the first time.
SR: Yeah…
Rudy: And uh…but that was a big innovation. A BIG innovation for Van Halen. And it made it…oh…I left… (long pause) I took eight Marshalls with me on the road last year, and all I…you know, all I used was one…basically, because I used the power amps. And I left, uh…four Marshalls home-I never even took ‘em on the road.
SR: Really?
Rudy: And I could’ve left another four…home.
SR: You…so you’d mentioned that the original head that Ed was using in the early years, he retired and is now for the studio…
Rudy: Right.
SR: …and you replaced that with that other one?
Rudy: Right. It w-…
SR: …that you guys found out there somewhere?
Rudy: Another…another old head that we found. Yeah.
SR: Where’d you find that? Do you remember?
Rudy: Umm… (long pause) I think we could’ve bought that on the road somewhere. I think we might have bought it from a guitar store somewhere out there on the road. We just…we kinda keep an eye out for ‘em.
SR: The remaining heads that you brought out, were those new heads?
Rudy: Umm…no, one of ‘em was a new head. But it had been modified…
SR: To the same…
Rudy: …to sound like the old ones, yeah. But, uh…the other three, uh…no there were seven of ‘em. I think three of them were new…three of ‘em, they were the new models and uh…or four of them were the new models and four were the old ones. But uh…uh…oh I just, I had a thought for a second. Oh yeah, interesting story about the old head. The very first year we went on the road, he not only took, umm…his original guitar with him back then in those days…before he built…the pre-Kramer days…but he also took the original, you know his “baby”, the amp with him on the road.
SR: Right.