In regard to the different SJ circuits, the truth is VERY likely that serial numbers after 03100 had the improved circuit. There was a tread in Dec 2007 called "Different Jubilee circuits???" Why is it very likely true ? Well back then the guy who started the thread posted the schematics officially from Marshall that looked like a blueprint which addressed the revisions. At the time I took a lot of notes, but for some unfortunate reason could not download the picture of this intial circuit vs. the revision. If you go to the original thread you can see the link is there to click on but it leads nowhere at this time. What I can tell you from my notes is that Marshall's official blueprint stated "2550 STD Also 2553 & 2558" for their initial circuit, and below that was the revised circuit that said "Models 2550, 2553, 2554, 2558" AND "USED ON MODELS WITH SERIAL NUMBERS 3100 ---->" ! ! ! Whoever posted the link added his note on the schematic "schematics from marshallschematics.com" Like the the link on the old thread mentioned, this URL does not work either but asks for a security check - don't feel comfortable going further. Hope that the pictoral schematic comparison is out here somewhere and that this post helps in some way. Earlier I thought from years back I actually had the picture, but I searched and only have only the notes I took on my newer PC. Gotta kick myself for not saving the pic I had. Gonna do a little digging and see if I can locate the schematics on a site other than the one above.