After 25+ years of owning-playing through combo amps only, I'm ready to "graduate" to the head+cab option. Coming from the combo setup, I've never had to understand matching ohms. So I am at the starting line...which to buy first? Do I buy the head at a "fixed" ohm setting and then attempt to "match" the cab ohms to the head? Or does this make any difference at all? I am presently looking at matching a GK 400RB-IV head with either (1) 2-12 Avatar cab OR (1) 2-10 + (1) 1-15 Avatar cabs. How do I correctly match these for max efficency?