There's really nothing exciting about variable impedance. The effects it produces are very slight and rarely desirable. Variable impedance usually means *lowering* the input impedance.
Lowering the input impedance means higher loading resulting in increased distortion for condenser mics, dynamic mics often show losses in the low end and/or low end.
Condenser mics usually show very little sound change, except capacitive coupled condenser mics whose low cutoff is affected.
Still, once you've made up for gain losses, there's hardly any sound change to speak of. Also, I have yet to encounter a mic that I liked better at lower than usual impedance.
All in all, variable impedance is mostly useful for a mic designer or tester. You can find out the mic's output impedance by measuring the gain loss. You can also find out, if your design works under marginal conditions such as very low input impedances. I tend to ignore impedance switches in real world use.
BTW. I had a chance to use the Vipre for a week or two. While it does look like a cool design, it is *not* very flexible. It has its own character, which you can vary a bit. But not in the way you might imagine. You can vary it from say 1920 to 1955. That's about its time machine range. To be honest, I prefer my much simpler A-Designs MP-2. I also think the Presonus ADL600 is a more desirable piece of gear. Haven't tried the newer GT Supre so far, but, since it is a simpler design (and stereo at half the price), I wouldn't be surprised if I liked it more than the big Vipre.
