The problem you are experiencing is not at all unusual. In order to hear stereo from the keyboard's onboard sound system, or secondary sound system, you must be sitting directly in the center of the keyboard and relatively close to the keyboard itself. The farther away you sit, the more you will tend to hear mono.
The same problem exists with most PA systems. There is a 45 degree cone of sound that most traditional PA systems have in common. Therefore, if the stereo PA speakers are placed 20 feet apart on stage, stereo is only heard by those audience members that are positioned within 20 feet of the speakers and in the center of the cone. All other audience members will hear either the right, or left speaker dominantly, depending on which side of the room they are situated.
Now, if I recall, the Tyros II does not have an onboard speaker system, and relies on outboard speakers. Try placing them farther apart, about 5 to 6 feet on each side of the keyboard. This should make a huge difference. Additionally, if you are using a crossover sub, place it on the floor directly beneath the center of the keyboard. You will be amazed at what you hear.
Hope this helps,
Gary
