Who of us has listened for a good length of time (as in months) to both a high end tube system and a high end solid state system? By high end I mean well over twenty thousand pounds worth.
You see, I have not had this priviledge and most likely never will. My understanding from reading what those that have done so have writen is that while the two devices have their strengths tubes ultimately have the advantage over transistors in transparancy and naturalness.
To be honest 'upgrading' is never ideal as really the best approach is to build it from the begining with components that were designed to work with each other rather than mix and match manufacturers and technologies. 
well... i am not sure how relevant it is, because i really do not believe that cost is the ultimate determinant to good sound. while i am sure that even a 50k pound system can be made to sound fine, i bet there are some 50k pound systems that are lacking. and i bet i could put together a system for 1/3 the price, that would sound as good as the 50k pound system that
isn't lacking.
but, for what it's worth, my system's retail cost was not far from 20k pounds retail, when i replaced my ($5k) electrocompaniet amps with the $250 asl wave-8 tube mini-monoblocks. (my cost, done incrementally, was far less than that, as i upgraded in interwals, & i almost always buy used.) and, while i have found tubes to be more natural
for me, it doesn't follow that it will be the case for everyone. and, i am a strong advocate of active crossovers, and for bass amps, s/s rules, imo.
and, i think it's just as easy, (if not easier) to find ultimate synergy with differing mfr's as w/same mfr's... yes, you have to find gear that works w/each other, but it doesn't mean you have to use the same mfr's...
ymmv,
doug s.