No you can't just disconnect it. It has to be rewired. there are a couple of ways to rewire it. One to keep the factory amp, and the other is to bypass it. I would have to hear it, but I suggest just turning the loudness off and adjusting the bass to a lower level. I have heard that radio (and many others) distort on systems that don't have a factory amp. The other thing to do is, make sure all of the connections are good and taped up properly. If you have any speaker wires making contact with themselves or anything else, at higher volumes, you will get distortion.
Just remember, when you change out the headunit and keep the factory speakers, you are giving them more power than they are probably rated for, so you have to keep the bass down, and don't turn it up all the way... It's bad for your ears anyway Good Luck