The basic sound will not change much on axis, although it depends on what the new effective baffle area will be around the tweeter. If there was nothing above or beside the speaker when it was standing up, and then you place it horizontally on and flush with the front of, say, a cabinet, there will now be much more baffle below the tweeter than there was, and this will change the diffraction effects and the response. Off axis horizontally, the new position will create a suckout in the crossover area as you move further and further off axis. That's because the relative distance of the woofer and tweeter changes, and that changes arrival time and relative phase. If the tweeter and woofer were in phase in the crossover region on axis, then eventually they will go out of phase with each other off axis, and the cancellation effects will cause a suckout. I've inserted two simulations of the S30 I worked with. One is the on-axis response with normal polarity. The other shows what happens when you reverse the phase of the tweeter. This is pretty much what would happen far off axis horizontally with the speaker on its side, except that the tweeter response would also trend down at the top as its dispersion narrows with increasing frequency. If you listen far off axis, you will hear the dip. On axis, the dip will be part of the overall power response, which includes reflected sound. But this may not be much of an issue depending on the room. Imaging might suffer, since the two stereo speakers will not combine in the same way in the horizontal plane. One thing for sure--it won't damage the speakers, and it's sure worth a try given what great bargains the S30's are.