If your dealer is an authorized Cary dealer and the pre does not have any cosmetic issues, buying a demo vs new does not present any disadvantages. The only thing that I would caution, is that the tubes may need replacing depending on use, but you will want to replace them with better tubes in the near future anyways. Dealer support, and the ability to audition, is worth a premium in my eyes, especially considering that you already have a good relationship.
Great preamp!
Great preamp!