use digital audio cable (coax) for subwoofer line-level cable?

Yes it is a perfectly suitable cable for a subwoofer.

perfectly fine

Education in electromagnetic theory is needed for a full understanding, but to try to put it simply:

Audio frequencies are much lower than RF frequencies. The electromagnetic wavelengths of audio frequencies are much longer than the lengths of the wires involved. As a result, you can transport audio signals over two separate wires with undetectable distortion.

In contrast, RF wavelengths are short compared to the lengths of the wires involved. Their cables have to be specially designed in order to avoid distortion of the signals. They're called "transmission lines." The cables have a center conductor surrounded by a cylindrical shield and have a characteristic impedance of 75 ohms.

Audio frequencies have no trouble traveling over transmission lines, but RF frequencies get seriously distorted if they are sent over separate wires.

ehansen42,

You're very welcome.

I should warn you that that discussions of the details of how much or little distortion is involved and whether or not it's audible invariably degenerate into shouting matches. A search of the forum will find many posts about cables.

Digital coax=composite video cable. They both have 75ohm impedance, and better shielding then regular RCA audio cable. Having said that you could use the flimsy RCA audio cable that comes with many audio devices, and use it as "digital coax", it will work just fine.

Years ago some audio old-timers decided to try using a wire coat hanger as a digital coax. They soldered RCA termination to both ends of the coat hanger.
There was no difference at all between the coat hanger and a reasonably priced "coaxial digital cable".

Now you've stepped in it.

I never heard the term "high-pass RCA cables" before. I wonder what it means? From the point of view of a co-ax cable, audio is about as low-pass as it gets.