How it works
As some of our readers are from a younger generation, not everybody will know what exactly why "vacuum tube" has anything to do with sound. You've all seen that old radio at your grandpa's house with some sort of lamp in it. Well, that's the start of vacuum tubes.
Here you get a nice example of one of those "lamps":
Compare it with the Musketeer's tube:
Same looks, only horizontally placed within the Musketeer.
For the more tech-kiddies, here's the explanation from Wikipedia
"The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal plate inside the envelope if the plate (also called the anode) is positively charged with respect to the filament(or cathode). This results in a current of electrons flowing from filament to plate. This cannot not work in the reverse direction because the plate is not heated and cannot emit electrons. This very simple example described can thus be seen to operate as a diode: a device that conducts current only in one direction."
If you have read the quote above five times and can not make heads or tails of it? Let me give you a more simple explanation.
A “vacuum tube” is in fact American English, it is commonly known as a valve amplifier. It is a device used for increasing amplitude of an electric signal, in this case: sound.
Today most sound systems use transistor amplifiers for economic reasons, but valve amplifiers remain popular for guitar amplification and for "high end" hi-fi systems.
The warm touch is thus given through that vacuum tube, turning digital into analog sound. If transistors would have been implemented, you could directly listen from your soundcard.