one disadvantage of valves is that they can sound over rich in harmonics to the point when it just sounds like a mush of sound, I have certanly noticed this with driven louder valve amps. From my experiences valves go muddy realtivley quick as well and start dying very soon! (sometimes sounding like gollum vomiting in a cave) buuuuaaaah, buaaaaah. lol. I prefer hybrid amps, like preamp valve's SS rectification, without sag, cos its punchier, and still not too agressive at the same time. I think Vai has got it spot on, I love his Carvin amp.