I am in the same process, with the same frustration. I have a shortlist of cans, and another of headphone amps, and they either cannot be heard together, or the amps not at all, before purchase. Let alone, combined with a dCS source.
My dCS shop has one of the cans, but not the amps. Another shop has all the cans, many amps, but no dCS gear. The shop with the proper head amps does not have cans ( !), but you can bring your own. And the totl amp is just made on order, no audition.
A shop in Amsterdam comes most close, and they ship worldwide: https://www.headphoneauditions.nl/ , but that is not going to help you in auditioning if you do not live nearby.