I went ahead and performed the first two modifications.
The first was to cut the wire running from the master volume to the junction of the preamp mixing resistors. It did indeed cause the master volume to max out at 10, as it should. So far, so good.
Next, I looked through my spare parts bag, and found some caps. To test the mod, I used test clips to put in a 250 microfarad capacitor to bypass the mid control at 0. He used 47 mics, but, as he said, larger values would make no difference. It sounded great! Lots more gain, and the mid control operation is better, less honky sounding. I went ahead and soldered in two 4 mic caps in parallel. That's only 8 microfarads, but I calculate that it will cause a rolloff at about 23 Hz. It shouldn't be audible, but it should reduce the amount of extraneous bass frequencies that get through.
I don't know if I'm going to do any more. The amp sounds so much better already, and he's optimizing for guitar. I play at least as much bass as guitar, so I may just leave it as it is. Or, I may move the tone controls to after the next stage. We'll see. I'll let you know if I do. As it is, I can highly recommend the first two mods.
