There are several differences.
First, the mid select. the way it is wired, the mid boost is always on. The control simply determines how much mid boost is present. But turning it all the way down does not turn the mid boost off.
The master volume is wired such that it maxes out at about 8, and turning it up beyohnd 8 results in less output. I have no idea why.
The tone stack is different. The first generation is a classic guitar amp tone stack, with the flattest position being with the tone controls at zero. All boost, no cut. The second generation has "hi-fi" tone controls, with the flattest response being at 5. Boost and cut, except fot the midrange control, as noted above.
That's my understanding, anyway.