Do I need an expensive digital cable?

@audphile1 said:

When you are using AES or coax connection to your DAC you are relying on a clock in your transport. USB connection would prioritize the clock in your DAC. 

This is correct.  Different set ups sound better one way or another.  If you want to use the (?better) clock in the DAC use USB.

@audio-b-dog asks:

Can anyone make a case that I should buy a "better" coax cable?

Yes.  It sounds better with a better cable.  At least it does to me.  I can hear it.  I actually prefer a better AES cable if I am using coax or AES output of the streamer.  Network Acoustics makes great cables.  Even better are Kubala Sosna.  Other good ones have been mentioned here, better easily than Pangea.

Note folks that recommend a better cable state they have tried them and hear a difference.  Naysayers point to an article or philosophy and say they do not make a difference.  I have never heard a detractor state they have tried one and can not hear a difference... I recommend trying one.

I noted a difference with better HDMI cables for picture clarity and sound.  That convinced me that there must be something that better digital cables do over the bog standard ones.  I can’t say I actually noticed an improvement with other digital interfaces but I think it’s reasonable to go up the chain a bit, but not into the stratosphere.  That’s my approach to analog cables as well 

@audio-b-dog the information you gathered from your dealer is inaccurate. With SPDIF connection, which is synchronous (aes/ebu, coax) the clock in the device sending the signal to the DAC is prioritized. Your transport is constructing the feed out to your DAC and that’s where the quality of the clock in your transport is critical. Give Simaudio a call and they will tell you the same thing. 
USB connection is asynchronous. But that’s not what you are using. 

Irrespective of the quality of the clock, the cable you use matters as well. As I said earlier wrong impedance will result in timing errors. Shielding, dielectric and conductor matter, as well as quality of the connectors. 

Your question was “do I need an expensive digital cable?”

The answer is no. You need a high quality purpose built cable. Major cable manufacturers mid tier should do as a starting point. Kimber I suggested earlier is not expensive but will make an improvement. You will be missing out on a potential of your digital front end if you are bottlenecking it with a wrong impedance and crappy quality cable. 

dh labs d750.  75 ohms.  If you got the impedance correct, length is less the concern.  1m is perfectly fine.

You are receiving good advice. A good purpose built digital cable makes a difference. The Kimber is known to be good, I have had good luck with Acoustic Zen as well.