A comparison between Fender and Marshall preamps tells a good bit of the story that you are looking for. Marshalls have smaller valued caps, particularly in the cathode bypass. The Fender runs about 25 microfarads, the Marshall runs around 0.68 microfarads. The next place to look is in the coupling caps...

Have a great day, Eric Pritchard, Pritchard Amps, www.pritchardamps.com