Well, excuse me!
Here's the problem. You probably do not want to insert a possibly mediocre piece of electronics in your signal line but, with a single-ended preamp output, that is your only option for changing polarity, afaik. Either (1) you use an active electronic circuit to derive the inverted polarity and a switch to select between the two or (2) you use a transformer for the purpose, again with a selector switch. Your move.
Now, if you had a balanced output preamp, all you would need would be a good 4PDT switch to swap pins 2 and 3 on each output.
Kal
P.S.: I have also used option 2 on the speaker lines but the switches need to be VERY substantial.
