Amelio,
Are you plugging the Tyros into a jack meant for phono, perhaps? A mixer going into the aux in will work better. This is how I rigged my studio monitors. I have a Mackie 1604 plugged into the TV/LD input of my stereo reciever and that feeds a pair of Event 2020 monitors. They're not the greatest monitors out there, but I wanted middle of the road/average gear to listen to so the mix would sound right in most home stereos.
I never use a sub in the studio because it's easier to make a stereo mix that works in a variety of speaker systems. I'd be willing to bet that most people crank the sub up to a level thats a little over the top as far as flat response is concerned. It sounds nice and full, and our ear responds to that. Nothing wrong, but when you mix for a sub and then playback without one ....... there is a big difference.

I have some news on the live rig sub I just bouyght, but I'll start a new thread for that.

BTW - I had to try a few times to get the driver to instal for the USB hookup too. Just follow the directions to a "T". It'll work.