all preamps gain and phase.

Low gain, typically.

Jafox: "So maybe it is only audible on certain music....or I am simply deaf!"

No, you're not deaf. Probably your listening skills are similar to mine--I can hear lots of differences but I definitely have a limit. Last fall, in a lengthy audition of interconnects, I could distinguish lots from others, but after some culling, those that passed all sounded the same--GREAT!

Each of us has a limit to our listeneing skills, and they're all different. Keep trying that signal-polarity switch; one of these days and recordings, you'll hear it.

One more point about preamp gain--linestages with high gain can create audible tube noise with high-gain poweramps. Understand that line stages amplify AFTER the volume control; that is, the volume control decreases the input voltage, not the output voltage, so the preamp's output noise is constant--unless a switch decreases it.
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Jafox--I gather the ability to hear absolute phase isn't like "normal" hearing. Some hear it, some don't. Think its called "Woods Effect" or something.