I have a mid 70's silverface bassman 100 with the matching 2x15 cab rated at 4ohms. my question is: what ohm rating should a 2nd cab be if i wanted to run it from the ext. speaker jack? i know that the super bassman could run 2 4ohm cabs (as it claims under each speaker jack "4ohm min" and you used to be able to buy it with 2 2x15 4ohm cabs.) BUT the bassman 100 only says 4ohm min by the main jack. i'm not sure how the outputs are wired but what i DON'T want to do is try to (accidentally) draw 2ohms from the head. i'm not an expert in this sort of thing so i'm hoping somebody can help me out. thanks!