Hi;
The 6B is quite similar to an early 7B, so the same rules apply. For low impedance loads (under 4 Ohms), you should use parallel mode, and connect 1 sub to each channel (1 red, 1 black per sub). For higher impedance loads (4 to 8 Ohms), use series mode, and connect both subs to the two red binding posts, with the left side to + and right side to - on the subs.
Short answer: Two 8 Ohm subs, use series mode. Two 4 Ohm subs, use parallel mode.
The peak/average switch would make this a custom unit, and this would affect the response of the LED's on the front panel, not affecting actual audio performance.
We made almost 400 6B's before releasing the 7B, so this number isn't out of line.
Mike