The N definitely got them later. The B-25B head, which is basically the same as the B-15S in nearly every way, had them. You may be right about the S. I've played through a few over the years but it's been awhile. Happy to be wrong here.
I wasn't trying to call you out here. The thought of high/low switches on an S didn't seem right. The 2 amps in the video are ones I have spent a lot of time playing through, though my 15N is a 61. Hearing them side by side I'm glad that I ordered the original Slampegg.
