i would think a speaker selector would be the way to go if your intent is to use both speakers at the same time & it would take away the chance of amp or speaker damage.
If you want something cheaper than the Bryston, Niles Audio also makes a switch box. My local dealer uses a Bryston switch in his setups. This is assuming you don't want to run both speaker pairs at the same time.
Your amp would probably be ok with both speakers in parallel but in general you shouldn't submit an amp to that because the speaker impedance halves. Since B&Ws are not efficient, it would be a tough load if all 4 are used simultaneously.
The Niles Audio offerings include one with some kind of autoformer impedence protection, as well as units without like the Bryston. The latter requires amplifiers that are compatible with a 4 ohm load. Niles units are indeed much cheaper than the Bryston.
Why would you want to? I'm sure there is a good reason to do this, but it escapes me. The amp will run a 3-4 ohm load but not at high voltage (high volume levels) for long periods of time. Impedence matching transformers will kill dynamics, so you may not care for the result.
Keep the volume down to a reasonable level and it should play fine. Neither speaker is a tough load for that Mac.