David Bray is THE MAN if you want a Marshall 1959 SLP brown sound amp. I've got one and it totally kills! I actually sent mine back to adjust the mod for current needs adding a bit more brightness than his typical mod. Also added a Line Out and a Power Amp in feature to power a preamp if needed.
Here's a clip of my amp David sent me for approval. Really captures that tone. Played thru a Les Paul into a mid 70's vintage Marshall 4x12 wich 25w rola Celestions.
My David Bray modded Marshall MP3