John, a lot of this depends upon how far apart the speakers are placed on either side of the keyboard. Additionally, with conventional speakers, the cone angle is usually about 45 to 50 degrees at best, so if those stereo speakers are spread wide upon the stage, the audiences will suffer with heavily stereo sampled sounds, such as Yamaha's Grand Piano and others. The fullness of the sound comes in the crossover area, which is the point where the left and right sounds collide on the dance-floor.

Many years ago, I saw an entertainer that had a pair of Eon 10s on stage. The speakers were 20 feet on either side of the keyboard. If you stood 20 feet in front of the keyboard, the sound was fine, but if you were at a table to the extreme left or extreme right, all you heard were the sounds from the right or left channel - not the combined sound. This was one of the many reasons I switched to the Bose L1 PAS when it first became available. I fed the left channel into channel 1 of the onboard mixer and the right into channel 2. The combination was just awesome. At that point I never looked back.

Using a "Y" connector essentially accomplishes the same thing as the mixer, but not quite as efficiently. If you plug into the mono output of most arranger keyboards, it does the same thing, only instead of using a "Y" connector, it does it electronically with an internal switching network. If you look at the schematic diagrams of the audio output of the arranger keyboards you can readily see how this is accomplished.

Now, stereo does sound a lot fuller, especially with heavily stereo sampled sounds - THAT'S A FACT! But, in order to make it all sound fantastic, those speakers should not be placed farther than a few feet on either side of the keyboard. This will provide you with the maximum crossover area and the best possible sound.

All the best,

Gary