You can use 4 speakers (2 pairs) on the Mac. It has Main and Remote sets of binding post. Impedance is not a problem for most Macs have an autoformer so it doesn't matter if your speakers are 4/8/16 ohms. Just like Aball said, you might want to check the tubes. Although most of the original tubes in the Mac 1700 were Telefunkens (including my Mac 1700), which are known to last a very long time it is better to check them to be safe.
The best thing about the Mac 1700 is that, 40 watts sounds bigger and more powerful than it seems, and it has a very good tuner section. On mine, I replace all output transistors with newer Motorola parts (Q36-Q39)and in my ears was the biggest improvement I did better than replacing all tubes. Do some experimentation, you will love this receiver. Enjoy.
The best thing about the Mac 1700 is that, 40 watts sounds bigger and more powerful than it seems, and it has a very good tuner section. On mine, I replace all output transistors with newer Motorola parts (Q36-Q39)and in my ears was the biggest improvement I did better than replacing all tubes. Do some experimentation, you will love this receiver. Enjoy.