In case you haven't seen it, this is a highly informative page about vintage Marshall circuits: http://marshall.redpt.com/clay/marshall_circuits_101.html It can help you in choosing/understanding the variations of the basic Marshall/Bassman circuit and the effect of different component values used in a given section of the amp. I originally said the following, but Matt H pointed out in a post below that the online text is technically correct. It is pretty accurate, with an exception or two... In the Slope Resistor section, it says: "You can even go to 16k for more bass and mid gain (and the overall frequency response flattens some)." With a 16K slope resistor, the frequency response does not get flatter -- it gets even more of a bass/mid-range hump. If you want flatter frequency response, go to 56K or higher.