Have a 1978 Concert lead head and 610L cab. The cab is 6 Ohms, but the output on the head doesn't say what it is. If I use a different cab, should I use a 4 or 8 Ohm cab. Does anyone know the Ohm's from the head? Want to use a smaller cab, say 2x12". 6 Ohm cab's are just not common. Thanks