It is a well known fact that vacuum tubes supposedly produce better sound than transistors---the music is supposed to sound warmer or something. I've never heard them in action so I can't vouch for them.
Sound is an analog waveform. Digital amps (Class B switched) will always have a slight edginess to them when compared to vacuum tube amps (Class A analog). It really does sound better with tubes, although in most environments you will be hard-pressed to tell the difference.
DUH!!!!! THey're freakin' HUGE. And with PC speaker sets (excluding the $150+ sets) there's no way to tell a difference.
The real difference when using tubes is the amount of amplification you can draw from them. If you want to fill a stadium with rich sound, you'll notice a huge difference in the range quality when you use tubes.