Mcintosh MA6100 ( how important is the 4/16 ohm) switch on the back?

I just started looking at the 6100 amps and have read that the later builds were updated with the switch on the back to cure some kind of speaker impedance issue?

So what is the big difference between the 2? My speakers are Klipsch Heresy and KG 3. If I get a earlier model what could possibly happen to the amp that does not have the update installed?

If I bought a earlier model is there a mod that I should have my local shop do?

On another note do these amps use Autoformers ? Do they ever go bad and can they be rebuilt?