The sound card should be able to remap the REAR(back) to be a second FRONT. This will let you run a cable to speakers and another to the sub.
This would need 2 of the best 3.5mm to RCA cables you can get and keep the cables as short as you can(longer cables can pick up some noise.).
Now this is Analog and some users do not like it, but its all analog when it gets to the amp one way or another.
As far as loading the sound card with a Y cable. It is very unlikely as you can connect headphones(lets say 32-64 ohm) to that plug without issues. The speakers are 28000ohm(very light load). Most computer speakers seem to be about 10000 and I can confirm that in my own testing, using to sets of 10k inputs on a y cable(so 5k because they are in parallel) did not lower or change the audio in any way(my split was only a few cm apart so it was a single cable(pair) upto that point).