Connect my amp out of phase?

Yes, that is all. Just in case you don't fully understand, they mean only at one end. You can either reverse the speaker wires at the amplifier end, or at the speaker end (same result).

Why is this? What is it about this preamp that makes this work?

There are a number of threads on reversing polarity. There is something called Woods Effect that some hear when polarity is reversed. Many people don't hear it. Whether a component (this is not unique to pre-amps -- amps, DACs, etc can reverse polarity, and things can be recorded with reversed polarity) reverses polarity just has to do with how the circuitry is implemented.

Your preamp is of a simple design, although unconventional, not something exotic. Most amplifying devices (tubes, transistors) change the polarity of the input signal. In order to provide the correct polarity at the preamp output, another gain stage (tube, transistor, or transformer) has to be added, which means more wires, devices and $$$. The extra gain stage is found in most all "conventional" preamps.

For abb@cs.com

The design of many amplifier stages result in inverting the
polarity. Your common everyday "op-amp" is most likely
inverting.

A preamp may have several stages - and if it has an odd
number of inverting phases - then the preamp as a whole will
be inverting.

Nothing really unusual about the AES preamp in question
here - except that the manufacturer leaves it to the
user to correct the polarity - rather than reverse
wiring some connnection themselves.

Dr. Gregory Greenman
Physicist