It seems to me that the Princeton Reverb is especially suited for mods of various sorts. A common one that I like is to use a Deluxe Reverb-sized output transformer. Then there is adding a mids pot or the Stokes mod. I am also a big fan of the mids pot. Or using a PT (and OT) that allows for running 6L6s or 5881s. Or even just putting a 12" speaker in a stock Princeton Reverb. Most all these that I've heard have yielded great results. Of course there is the discussion of whether a Princeton Reverb with a different output transformer is "really a Princeton," or whatever, but that is not the point of focus here. Historically speaking, these sorts of things formed the aegis of things like the Paul Rivera Super Champ or the Randall Smith Mesa-Boogie line. And this is particularly interesting, since the Princeton Reverb is the blackface/silverface amp that shows the most direct tie back to the tweed line, but it seems also to have offered the possibility of being the most forward-looking at the very same time. So what, in your opinion, are the key features of the Princeton Reverb's topology that has historically made it such a great platform for modifications like these, even leading to a whole new line of amplifiers in some cases?