Connecting both actually increases output by 6Db or something. It shouldn’t cause this.
Or just buy sub or subs in the first place that don’t need any extra output cable nonsense. 
Just Troubleshooting one thing that baxman293 could try. Single RCA from tuk’s to sub like instructed and guided by Kanto.
They usually work.
just tried it, doesn’t work
So I don’t know if we are missing something or what but…
How are the Tuk’s connected to the computer ?
How do you connect the sub to your Tuk’s ?
Does your pc have optical out ?
Need these answered in detail then there might be a simpler way to help.
Also judging by the pic of the sub you had RCAs connected, what is on the other end? Hopefully not the Tuks. Please inform…
Also a pic of the back of your motherboard sound out or whatever you use would help.
I don’t have Kanto Tuk speakers, but I do have their YU6 powered speakers, along with their Sub8. Since the speakers already have a DAC, an external DAC would be superfluous, and won’t work (would probably screw things up to boot so you wouldn’t hear anything). As for the subwoofer, I believe you need to connect one that’s independently powered (like the Sub8). And, if my setup is any guide, you only need to connect one RCA plug at both ends, even if the powered sub has inputs for two (as the posted guide shows, either L or R should work, it doesn’t matter which).
I’m away for rest of the week will do some more info when I’m back