As to the specific mods to the stock JCM 800 preamp----- most of the info is contained in this 111 page thread at the Metro forum: http://forum.metroamp.com/viewtopic.php?f=3&t=29759
There are layouts of many different builds of different versions of the three basic Slash/AFD mod amps: #34, #36, and #39---- these numbers orig identified the stock # stenciled to the ends of the rental amps owned by SIR studios, they owned and had their in-house techs (Tim Caswell and Frank Levi) mod some rental stock Marshall amps back in the 80's.
Over at the Metro forum there has been this on-going project to re-create the tone of these modded amps--- i've used what they've provided and added my own changes on some of my five experimental amps as i've seen fit.
The #36 and #39 Mods have an extra preamp tube----- the #34 Mod does not----- there are detailed layouts of several versions of #36 and #39 mods---- there is no detailed schematic/layout of #34 Mod because they say Frank Levi (tech who did this mod--- Slash still owns the orig amp and it was used as a base by Marshall for their AFD 100 amp) wishes that this detailed info of the #34 Mod be kept "secret" (even though there is enough info out there to figure it out).
None of my 5 experimental amps matches any schematic/layout exactly --- mostly because all the orig amps were 100 watters and none of mine are---i've also used several different approaches on my amps to see how local players react to them, trying to get useful input (as in:What one(s) do you like best? and why?) from players who demo my amps- i tend to like whatever one i'm playing at the moment the best so input from many different players is valuable to me---so far i've gotten a lot of "they all ROCK !!"---- which is great for my ego but doesn't offer much info from an amp builder's point of view.
You, or anyone else, are very welcome to stop by my workshop to play and give opinions of these amps--- i'd welcome the input !!!--- and i'd be better able answer questions on how each amp is modded...........................gldtp99