Hi Squeak

Here's a possible reason for you thinking the tweeter doesn't work. This applies to the 740 but is probably the same on the 550. The fundamental frequency of the highest note on the keyboard is 2088Hz (for the piano at any rate). The crossover frequency for the speakers is 6000Hz which is well above the fundamental. So you'll only hear harmonics of the highest note from the third upwards from the tweeter. If the sounds aren't particularly rich in harmonics, you won't hear anything. Middle C is 261Hz so you'll be trying to hear the 23rd harmonic. Incidentally, I can hear the tweeters on the 740 quite plainly but only barely on middle C. Much more plainly on top C.

Bryan