I have a Tyros4...I don't recommend using one Bose L1 with it...the T4's stereo (especially piano) doesn't collapse well to mono.

I have recommended that customer with a mono PA system also bring a small powered speaker to plug into the Right output. With the PAS system you wont need the speaker for personal monitoring, but try placing it in different locations to find what sound most pleasing both to you and your audience.

The problem with using a mono output on a stereo sample is the cancellation that occurs when 2 identical waveforms that are out of phase with each other collide. The second speaker does not have to be of equal volume with the PAS. It could be, but it needs to be at least loud enough for YOU to hear it. That way you are hearing both sides of the sample. It would be ideal if your audience could hear in stereo, but that just isn't going to happen for many people. Play a MIDI file while you walk around in front of the stage. Try placing the the secondary speaker in various locations and at various volumes. If it doesn't sound better from the audience perspective, point it at your right ear and turn it up just loud enough to blend with the PAS.