okay, if you insist.

No difference. The 6EU7 was a 12AX7 in a noncompatible pinout. NOS 6EU7s were like comparable 12AX7s of the period, they had better cathodes and closer quality control than the stuff currently being manufactured.

Any major sound differences are due to circuit design variations.

Don't believe anything you read on an eBay listing. It's like Audio Asylum---incoherent ravings and outright lies are commonplace, nobody tells nutcases they can't say whatever they want to sell the item.