preamp inverts polarity

The speaker cables are marked differently so you can get both speakers wired consistently.  They are not made of different materials designated only for + or - . 

Are the tires on your car for only going forward?   Reverse the wires and be done with it.

genez, the positive and negative cables are different as i posted in an earlier post (4th post)
" Grost chose to construct the negative/ground cable from a mix of conductive composites and wire. The positive lead according to the literature, on the other hand, employs hundreds of stands of microfiber saturated in proprietary liquid ceramic.

"Is "phase inversion" the same as "polarity inversion"?"

Good question.  

From the intro to The Wood Effect (1988) by Clark Johnsen, audio journalist at large,

Newcomers to Polarity do deserve some introduction though, which for the sake of everyone else I shall make brief. The abstract to The Wood Effect (1988) still explains it best:

Masked by random combination with other distortions in the music reproduction chain, an unsuspected major contributor has lain hidden: Aural sensitivity to "phase inversion", the Wood effect.

Musical instruments normally create compression waves in their attack transients. Electronics, however, often invert that natural, positive polarity to negative, unnatural rarefaction as emitted from loudspeakers, thus diminishing physical and aesthetic impact. The term Absolute Polarity uniquely describes the correct arrival to the ear of acoustic wavefronts from loudspeakers, with respect to actual musical instruments.

Wrong polarity, when isolated, is obvious to almost everyone. Its present neglect results primarily from habitual disregard for linear phase response in loudspeakers, due largely to the erroneous auditory theory of Helmholtz.