How thick/heavy/stiff is your coax cable. On heavier and stiffer cables, it puts stress on the RCA jack itself. What happens is it bends the internal contact as the weight or stiffness of the coax cable applies tension. At some point, the internal contact of the RCA jack becomes "out of bounds" and a standard RCA cable inserted will not make contact.
Is there a difference in sound between a coax cable and an optical one?
I've had 2 separate DACS within the past 20 years, both using a coax cable. I've had trouble with both with the connections. They either come loose or disconnect or whatever. I've always had to go and wiggle the cable to get the sound to come out again. The older one was an Audioquest cable (long gone) and the one I'm using now is an MIT (probably 12-15 years old). My plans are to get a new pre-amp with a built-in DAC and if there is no substantial difference in sound, I plan on using an optical cable this time around. Any thoughts?