The first function is a simple source follower. The "Power Scale" pot sets a voltage on the Gate from maximum to near zero. The MOSFET Source puts out (essentially) the same voltage, except at very low impedance instead of the high impedance of the pot.
The Zener just protects against gross abuse, won't normally do anything. You could leave it out and the circuit would work, until the dry winter day you shuffle across a wool rug and throw a static spark into the pot and thus to the Gate.
The 15 ohm resistor and BJT transistor form a soft current limiter. When the resistor voltage increases enough, Q14 turns on, pulling down the Gate voltage and reducing output voltage. Class AB guitar amps vary in sound as the increased supply current on loud sounds interacts with power supply sag: this allows the hard/harsh sound of a hi-fi amp or the soft sag of an early Fender.
D3 seems useless. D2 may be needed to save Q14 from the 1uFd output cap when you turn off the amp. The 15, 33, and 1 ohm resistors all carry the same current and should have power ratings proportional to resistance; it is odd they don't. For one pair 6L6GC, they could all be 2 watt.
The 953(???) resistor keeps Q14 Base from burning up, and could as well be 1K. The 100K looks like an attempt to skew the Sag pot center setting, but is too large to do much; would certainly work without that. The 33K at the bottom of the 1Meg pot sets the minimum output to about 20V; otherwise you'd have to turn the pot to 1 or 2 to get any output at all. Not essential, just nice.
At minimum setting, a Fender Twin would be down to about 80dB SPL at "full" power, a level you could easily talk over.
But wait. This trick won't work right on Fender Twin or any fixed-biased amplifier. Turn down the G2 voltage without altering the G1 bias, and the amp will just cut-off. It will be unresponsive except at maximum input. There may be a musical style where this "works", but it may just sound like the kind of bad connection that only passes the loudest signals. It should work fine on any self-biased (cathode resistor) amplifier.
The MOSFET should be heat-sunk for about the amplifier's maximum output. Pair of 6L6GC with 500V, you need a 50 watt heatsink. This could be the heatsink from one channel of a 100 watt transistor amplifier. This will generally be a sizable addition and packaging problem.
Use the very best insulator on the MOSFET and remember that all these parts, especially the MOSFET case/tab, live at fatal voltage.