i have a swr sm 400 bass head swiched to stereo mode 250 watts one carvan 118 cab w 2 1/4in jacks on input jack reads impedence 8 ohms jack 2 marked extenion and at top says 4 ohms @ 600 watts cab 2 has no markings it has 4x10 evs whats the easyest way to calculate impedence and ohm...