I traced out the SD and its pretty different from any other amps for the most part but it seems to be based off the mid 60s Marshalls but without the extra fat
I mean its not dead on by no means but thats the closes to any marshall it resembles based on the tone stack and a few marshall values. For the most part Jeff and Dave Friedman (from rack systems) kinda did their own thing when designing it which is really cool and unique.
Speaking of Dave, can anyone fill me in on the story how Dave got involved with designing the SD?