Jack don't worry too much about the specifics, just know that a Zu speaker will never drop, and will stay a stable load, and as for 4 or 8 ohm taps thats fine with a 12 ohm speaker, via tubes it is different than solid state, the 8 ohm taps would most likely sound best and run just fine, also most tube amps don't change power output at all between the 4 and 8 ohm taps, if its a 40 tube its 40 at either tap, normally a solid state is 40 w at the 8 ohm for example and 80 w at the 4 ohm...
But even solid state amps like 12 ohm loads, but might not be optimal all the time I would assume cause your not getting the full juice out of it cause it never has a reason to break a sweat, but I don't know, some amps are just better being driven harder, Class A tube amps are full output all the time even not driving a music signal thats why they probably sound the best, but then again thats why they run hotter too.
But even solid state amps like 12 ohm loads, but might not be optimal all the time I would assume cause your not getting the full juice out of it cause it never has a reason to break a sweat, but I don't know, some amps are just better being driven harder, Class A tube amps are full output all the time even not driving a music signal thats why they probably sound the best, but then again thats why they run hotter too.