Recommendations for a good audiophile digital coax (BNC-BNC) 50 ohm cable?

I am also a firm believer in Wireworld wire, use their BNC terminated cable between my Esoteric player and word clock.
You can also try cables from HAVE, Inc., they have standard ones as well will make up special cables for you with any ends that you may need. They use both Mogami and Canare wire products.

Oh, I see the Mutec has both 75-ohm and 50-ohm outputs; I’d guess the K1 has a 50-ohm circuit, so Mutec suggests you match it.

If you bought one of these Amphenol cables (almost free) you would at least have a baseline for assessing the boutique ones. Amphenol has been designing and selling radio-frequency cable and connectors for generations for commercial and military use. It’s hard to imagine that a specialist cable manufacturer working out of a small shop knows RF technology better. With such a cheap baseline, you can decide for yourself (you might want to ask Mutec their opinion -- they are sensible people).

Thanks guys, indeed several options to go for. It seems (reading between lines) that matching the 50 ohm impedance is more critical than getting a "beautiful looking" audiophile cable, so it might be my best bet to go for a baseline one then stay tuned for a more audiophile for future upgrade (I will indeed try to confirm benefits of such upgrade with Mutec).

rajah:
I have the same gear and initially used a $20 2-meter 50 ohm cable I bought somewhere locally, which sounded great (much to my surprise). I could probably have lived with this set up for quite some time.

I ultimately bought the Shunyata Sigma 50 Clock Cable (which is very expensive), and that made a huge difference (much again to my surprise).

I also use the 75 ohm REF 10 output to the Mutec 3+ USB input to use the REF 10 clock as a reference for it to clock my DSD recorder through the 3+ USB at 44.1 (as recommended by Tascam DA-3000) to digitize analog recordings (I use SOtM 75 ohm cables for that purpose).