In regard to the S5, I can give some real world experience with vibration and backwave distortion from the design. I have noticed that I can feel vibrations on the speaker (side, top) when playing music, specifically rock at higher volumes. At first I was a bit surprised at this since the S5 weighs almost 200 pounds. However, it did not appear to be negatively affecting the sound. In fact, I found the sound was so good, clean, and precise that one night I just turned the volume up higher, higher, and higher. Finally, I was maxed out on the preamp, and I was literally being POUNDED by the SPL level. I just sat there amazed that a speaker could play so incredibly loud, and still sound so good. Perhaps there was at bit of distortion at that insane level, but it certainly was not apparent at the time.
While I have no intention of duplicating that feat, I am quite confident the cabinet design of the S5 goes far when it comes to eliminating distortion caused by cabinet vibration.