With the 2A3-40, you can follow the instructions for wiring the FC-1 for 2A3 operation and hooking it up to the power supply for the 2A3 version, then go with 300B wiring for everything else. This will give you substantially more power with the 2A3-40, but will make the amp incompatible with those Sovtek 2A3's. (The 2A3-40 is a 300B with a 2.5V filament)