I'm a little surprised that the manual for the Poseidon has nothing to say about altering the filament setups on each socket. I kind of wanted to post in part to make sure my thoughts are correct, and in part to leave this info floating around for anybody else who purchases these. On the 12AX7 socket, orienting the socket with the pin gap at the "alt" pin, the board can be left alone. The triode 1/2 using pins 1, 2, and 3 will be heated, the other left cold. Rotating the socket so that pin 9 of the socket is moved to pin 4, the 'x' on the board needs to be removed to prevent the first triode 1/2 from heating. In this case the 2nd 1/2 of the tube will be used (pins 6, 7 and 8). Please confirm that this is correct, or add any additional info.
-Paul