Knowing that we all have different ears, ear lobes, frequency curves, preferences, needs and budgets, knowing that there are about half of infinity of audio components' possible combinations - how does a manufacturer set up its own "reference" listening rig ?

That is the realm of Ryuichi Sawada.
Mr Sawada entered Marantz in 1977, just when the Marantz "A Line" was about to end. The A Line was that comprising the 100% USA-designed components (the last of which was the Sm 1000) while the B Line was all the rest, designed and made in Japan ; from 1979 onward, the B line was... everything, although quite a lot came from Philips until the late 1980s.

Of course, rigs, listening and "references" rooms changed throughout the double triple ownership (Superscope / Philips / Marantz Japan then Dynascan / Philips / Marantz Japan), double ownership (Philips / Marantz Japan), the very brief single ownership... and the (now soon to be over) D&M ownership.
So here is most of what Marantz in the USA, then in Japan, used as reference audio systems and the related audio sources and main focus of attention(s).