I want to wire two sets of 8 ohm outdoor speakers and need help with an impedance question. I intend to wire to a A, B, and A+B selector switch. Then wire an impedance matching volume control to the A set of speakers and an impedance matching volume control to the B set of speakers. At times the A set will be on or the B set will be on, and, at times, both A and B sets of speakers will be on. What impedance setting should I use?