Martin Logan SL3's

Why does the piano spread itself ALL the way across the sound stage on these speakers? Why do I sometimes hear so much more coming DIRECTLY from the panels while the center image is much further back? I don't experience this with my Klipsch speakers.
Wow, how to answer this question without writing a book about speaker design and set up. :-)

Generic panel speakers lack a 'point source' driver for the high(er) frequencies and the high frequencies radiate off the entire surface of the speaker which creates that spread you describe. Delivery of high frequencies from a small source is critical to getting exact instrument location and relative size. Some of the old panels which have neither electrical compensation or added tweeters will expand a singers mouth as much as you experience with a piano! :-)

Some panel speaker have added line tweeters (like Magnapan) to combine the benfits of a panel but by way of the addition of the line tweeter added a source for a specific high frequency sound which will add to the sense of both a large acoustic space and also some specificity when it comes to placement of individual instruments.

Horn/cone based speakers thrive on their ability to deliver specific imaging (pin point) but potentially give up some in the creation of a large soundfield.

Both require careful set up to deliver what they do best AND, more importantly, improve on what they do worst.

You are hearing the seperation (location of sound to each speaker) most likely due to improper set up. This can happen with any type of speaker. But panels are especially vunerable because you have not only a frontal (direct sound from each speaker) wave of sound to deal with you must also factor in the effect of the back wave which bounces off the surfaces behind the panels and has almost aas much effect on the overall sound.

In a box enclosed cone/horn speaker there is no back wave, so the sound field is only minimally changed by set up issues related to surfaces behind the speakers (other readers note I'm NOT saying it doesn't make a difference - depending on your room, your listening skills, your expectations, and your speakers it can make a large difference!)

Hope that helps a bit.