Here's a crude schematic I drew. It's odd that the tone controls are in the cathode circuit of the 12AX7 and one of the leads from the extra winding on the transformer goes back to the tone control. If this was a handwired amp, it would be much easier to modify it. However, since it's a PC board, it's going to be harder to deal with. Leave it to Sears to come up with some goofy circuit design.