I know what you're saying but two 8 Ohm speakers will still show the amp 4 Ohm. So even though they are independent in the cab, when you connect to the parallel outputs of the head they'll be two parallel speakers giving 4 Ohm. There's a chance your head is 4 Ohm but I doubt it seeing as it's a plexi copy. I'd say you're better leaving the cab at 16Ohm and working away with that rather than risking damage to the OT.