When you see an output transformer that has a four wire secondary setup with 0, 4, 8, and 16 ohm connections, you end up with half of that winding unused when using the 4 ohm connection. This is not particularly optimal, and in a zero global feedback amp, it's possible to design the transformer secondaries so that either all of the copper or nearly all of the copper is in use regardless of the speaker impedance configuration. For this reason, our iron is a little more cumbersome to wire on the secondary side, but that extra effort is rewarded sonically.
From my personal experience, I have yet to run into a 16 ohm speaker that could really make use of the extra juice. I would just leave the amp on 8 ohms and not worry about it. We did at one point offer a PC board that mounted above the OT-2 and offered impedance switching capabilities, but the switches used were frequently out of stock and have since been discontinued. I seem to also remember that at some point PJ drew up a switching configuration using a 4P3T switch as well.
