4ohms or 8ohms for refurbished WATT/Puppy 5.1's?

The 4 ohm tap has half the copper windings of the 8 ohm tap. Use whatever sounds better.

The rule of thumb is to try them. Like positioning, it is a suggested starting point.

i would start with the one you immediately think sounds best and leave for a couple months. This will allow you to really get to know the sound. Then switch. If the change is not obvious, leave there for a couple weeks and switch. Doing really slow changes will allow you to listen to the music… all kinds and really know the sound before changing. It is likely the best will stick out like this.

Thanks everyone!  I was unsure if it would potentially cause issues with the MC2100 but sounds like it won't.  I am going to keep it at 4ohms for a while and listen before switching back to try 8ohms, since I've had it on 8ohms since I got it all working.

It does not matter, as far as safe operation is concerned.  The 4 ohm tap would probably work best with difficult/demanding loads and almost all Wilson speakers are that kind of load regardless of their efficiency.  

@OP. I'm not sure about the 5.1, but the original series 5 had a couple of severe impedance dips in the bass, including one at 100Hz where there is a lot of energy. But it also had a rising impedance from 5K onward, so it's idiosyncratic. So the best bet is to listen to both taps on a wide variety of music and see which sounds best.