Nik turned me on to this great mod. I replaced the standard volume pot with a 1M push/pull pot with two solder lugs:
I borrowed Hoffman's pic because I bought the pot from him.
If you need parts quick, he's great.
The 15k resistor that's soldered to the back of the original pot is lifted and soldered to one lug of the new pot. The 3 upper lugs are connected as per stock. However, an additional ground wire jumps the pot's ground to the second solder lug:
In position 1, the amp is very clean and has less volume. In position 2, it's big and fat with extra gain.