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

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).