Are there differing qualities of optical cable?

There are differing qualities of optical cable, yes. Glass cables are generally better than plastic cables (almost always true). ST optical connections are better than Toslink optical connections. (almost always true)

Coaxial RCA cables for digital are almost always better sounding than any optical cable. For audio (A Caveat).  As in-I've never heard a optical cable outperform a coaxial connection.

the right inexpensive coaxial cable can sound better than some expensive ones.  Try a few, even some well built cheap ones.

Your experience with the optical cable seems to be saying that something is likely amiss, as George is no dummy. His coaxial cable will be well built and sound good.

Your situation may be exactly true and right but it would be very much against the odds that the record of experience indicates as what is likely to happen..

@rvpianos 

To add to what teo_audio has stated regarding coax, I am using coax from my Oppo to Yggy. In fact, Schiit recommends coax (over optical). I asked :)

That said, all of my cables are from https://www.bluejeanscable.com/

It shouldn't sound different unless the DAC is not handling jitter very well in which case you may need to experiment to find out what works best in your situation.

I assume you are playing CDs which should be fine. Just avoid using this DAC for any higher than 18 bit files as it probably just truncates and throws out the least significant bits on any digital music above 18 bit.

Since the consensus seems to be that coax is better than optical, I ordered the DH Labs D-750 coax cable.
Hopefully it will do an even better job than my Monster optical cable, which is sounding quite good right now.