A simple adapter should do the trick
and I would prefer XLR cable over coax cable for long distances.

SPDIF inputs can not be destroyed that way. The slightly higher voltage allows longer cable runs and also helps (some) insensitive inputs to work reliable at 192 kHz.

This is the confusing part  

• Some say "just run XLR to RCA" straight in over any cable and SPDIF input will work within <5m.

• Others say you absolutely have to pad the voltage down to under 1Vpp.

• Others still try to match impedances with a Matching Transformer and pad voltage down (probably only relevant for long cable runs).

Like in Figure 1 here: https://www.ranecommercial.com/legacy/p … _SPDIF.pdf

Even more, the XLR-to-RCA digital cables can have different pinouts:

• in the Rane document, Figure 1 shows to connect XLR ground+shield togehter (via a resistor?) to output to RCA ground. This is also similar to RME manual for RCA-to-XLR digital AES cable

• While RME manuals say that for XLR-to-RCA digital cable, the shield is lifted (not connected to RCA) and only XLR "ground" goes to RCA ground - this RME suggested cable sends full differential voltage to RCA SPDIF - not ideal.

• Finally following the Rane analog interconnect manual https://www.ranecommercial.com/legacy/p … ection.pdf : it is suggested to skip the XLR "ground" pin and use XLR shield as the ground for RCA - thus reducing Vpp in half (getting Voltage closer to nominal SPDIF RCA standard)

Who to believe?   My best guess is to use 2.6 Vpp setting with a cable where I skip the XLR "ground" pin and use XLR shield as the ground for RCA. That should lower Vpp down to almost nominal 1.3Vpp - then hope for the best?

The logical alternative XLR-to-RCA digital cables is to connect XLR ground+shield togehter - that would be my first guess, but for some reason RME manual does not suggest this.

Did I mention that this is confusing?

It depends on RME device. For certain RME devices manual says "shield lifted" for others "shield grounded". I have asked about it on this forum, and RME (probably MC, but it could have been other RME guy) explained, that it is indeed different for different RME devices. That they test what works best for given RME device....

And concerning the levels. I do not care, I just use similar adapter like suggested in this thread and no problem. What about just try the adapter? If it works reliably for you, good, if not then I would seek another solution.

The only thing to solve is, whether to use XLR cable or coaxial SPDIF cable. If I was at your position, I would try that cable that I have handy (for that length) - XLR cable. If that would not work well, I would try coaxial one.