I'd be looking for a 16 ohm. I'm also assuming the G12's are 16ohm as they're out of a jcm900 2x12 combo. Perhaps someone could correct me if I'm wrong as they're not marked.
is it a 50 or 100 watt JCM? Would you be interested in some sort of swap/part exchange with an AC15C1? I also have a Vox extension cab with a vintage30 in it.
This won't help with buzz but I recommend getting and RCD to plug your 4 way into also. They're mega cheap and very life savey, especially in unfamiliar venues.