With my understandings & experiences about subs, a thick pure (99%) copper cable (1.5 or 2.5 mm) like Finolex will do a nice job, as low frequencies are not that vulnerable to interferences like high and mid freq, at least not audible. So HQ mid thick electric cable will do the job. Have a try, you will not regret.
I think he needs coaxial cable as the subwoofer is active. Rupam, I'm using belkin two meter digital coaxial cable as subwoofer cable and I haven't experienced any quality loss.
Like other FMs here, I am using a Bandridge coaxial cable for my sub and believe me, its fast, crisp and tight.
Subwoofer cable is just like an interconnect between the source and amp but its not as significant since you will not be able to hear the nuances as much as you can on the full range audio transfer. Expensive subwoofer cable makes sense only if your receiver and sub are expensive.
Currently I'm using a ordinary RCA cable for my Denon, will take your advice and use a spare DAC 75 OHM shielded digital coaxial cable. Lets see what happens.
I use a pair of 12 meter Monster 600sw cables to connect to the Rythmik sub. No ground loop or hum issues even with the long cable run. Its shielded pretty well