So, you disconnect your front speakers and hooked up your subs with that output from the radio?
This is not a good way to do things. You should only be sending your subs low frequencies. You are sending everything to your subs which isn't good for it. Also, your head unit probably can't power those subs correctly even if you were sending the correct frequencies.
Temp fix would be to go get a low pass filter and add that before the subs. This will at least protect your subs from the mid and high frequencies.
Best fix, wire back up your front speakers. Get a small amp to power the 8 inch subs. A good amp will have a built in filter. Usually you can select low, mid, high.
If you have any questions about how to wire up an amp, just post em.