Ok, let me start by saying that i already know about having to match the impedance and how to work it out etc. etc. however:

On the back of my JCM900 it has a switch labelled '4 (16) - 8', and obviously i know that this means if i flick the switch to the left, i have 16 ohms selected, and to the right means 8 ohms. Now, there are two jacks for cabinets, so if i have an 8 ohm cab plugged in each, do i need the switch set for 8 ohms, or 16?

I'm thinking its 8, but some clarification is required for me to be happy

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4 ohm on the amp for 2 8 ohm cabs

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Your head can do either 8 or 16 ohms and that's it.Here is what the manual states:
heads:8 or 16 ohms
combos:4 or 8 ohms

The number 4 should just be ignored.They put it on there for no reason basically.The idea behind using two cabs is you must divide the impedance in half.So two 8 ohm cabs=4/two 16 ohm cabs=8 etc.You generally don't see guitar amps with a 2 ohm option,so running a couple 4 ohm cabs would not be possible and you won't see many of those anyway.

If you try to run at 8 ohms with both cabs you will probably blow the output transformer or at the very least it won't sound very good.With this head you can either use two 16 ohm cabs or just one 8 ohm or 16 and that's it.What cabs are you trying to use? Marshall 1960 4x12's are always 16 ohms,at least the newer stereo ones.Try to get a few of these or borrow some,or just run 1 cab only.Don't blow your amp up...it's costly!

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Im running a 1936 2x12 and a home-made ('custom') cab, thats an open-backed 2x12, each running in 8ohms mono. Yea, i knew about the number in brackets applying to either the heads or the combos. Cheers for the info, 1 cab it is then

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