The guy is asking what we think sounds best. It's a bit expensive to buy every type of connecting cable and try them all. I'm sure he'll narrow down his options from what we suggest to get a better understanding.
"What sounds best" is an impossible question to answer. What is "best" for me most likely will not be "best" according to your expectations, or will be a very different "best".
Here in Europe it is possible to loan cables from dealers. You give them your details, copy some ID, sometimes give them some security money, and you can go and play with them at home. In the end differences between cables are not immense (in spite of what some state), moving a loudspeaker cabinet a few inches has considerably more effect.
Naim's use of DINs isn't exactly the norm. I've heard Naim systems using RCAs and DINs and can say to my ears DINs always sounded far better than RCAs. Not sure about the XLRs vs RCAs on the BDA-1. If the XLRs sound that much better than the RCAs, maybe an XLR to DIN cable would theoretically sound best, all things being equal.
This is a daring thing to say... Have you done a direct A/B comparisan? As in connect both cables and switch back and forth while continuously listening to the same source with the output level corrected? If the answer is "no" then the difference you are talking about must be truly considerable, because human memory for sound is quite poor.
The guy is picking our brains. Nothing wrong with that. Have you ever asked a question before? If not, how did you obtain all that knowledge that you seem unwilling to share right now?
Yes, I have asked questions. Lots and lots even, including here. I have also experimented, sometimes with costly fails (like the BDA-1 which I sold after only a week of listening to it). I also read reviews and am present (either active or just lurking) at several forums. Whenever I have my mind set on something I will learn everything about it there is to know.
As for DIN, it is a seventies thing. In the end, as long as a signal gets from A to B it doesn't really matter what sort of plug is used. As long as it is properly shielded, and has a decent cable connecting the A and the B.
My 2 cents: find a place that will loan out a bundle of different RCA cables and see what is liked best. Sound is personal, I have cringed several times at the sound of people that were convinced they had better-than-studio-quality in their living room.