my understanding is this:
tubes 'distort' differently than SS (and EVERYTHING distorts. it is just, how, and to what extent, how/if you can perceive it, etc) - something to the effect of the distortion being in phase w/ the output?
therefore, when it comes to sound stage / detail (which is typically, at least in my mind / ears, derived from the highs/mids) if you are distorting in a more 'pleasing' manor, you will 'hear' better imaging.