preamp inverts polarity

Are the speaker connection non-standard?...  If they are spades, just reverse the pos and neg.

It certainly does not defeat the design of your speaker cables.  Just reverse on BOTH speakers.  I had 4 tube preamps which inverted polarity and never had a problem with the speaker cables.

the negative cable has little different design than the positive cable, they are both designed to be connected to the negative or positive terminal. the connections are both standard just the internal design of each leg is different.
these cerious technologies speaker cables, here is a better description from a review.

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.