I always assumed (?) they had the real amp opened up and measured it component to component (?) ....... if they did it solely from a schematic how would they know if it sounds like the real thing if they dont have the real thing on hand to compare it to (?) ... and if they have the real thing anyway, surely measuring the real thing the first time would be the best / most ideal way to go (?)
By way of example ...... some 18 months or so ago I was banging-on about the missing EF86 Channel of the Matchelss DC30, and from memory, Cliff (?) chimed in to explain that they no longer had that amp and that the multi-stage "tone" control would necessitate a lot of modelling / measuring .... and this is despite the plethora of DC30 schematics online ...... so I suspect the real thing is needed if not critical
BTW ..... did I say real thing enough times
Ben