If you know anything about tubes, you know they weren't originally made for outrageously priced audio gear. Despite a number of tube houses now pumping out audio-specific glass, there's a whole slew of products using bizarre tubes from all kinds of backgrounds, most of which are completely mundane.
The GREATech muVac's tubes, however, have to be from the most interesting background of all -- WWII artillery shells. Yeah, back in the day, shells were equipped with metal detectors instead of impact detectors. The idea was the shell would sit in the ground until a tank or whatever rolled across and then the little tubes would go into action and cause all kinds of fun.
Of course, we're not using these tubes today for our missiles so, naturally, these tubes are in limited supply. Since they were made for, you know, firing out of cannons, they're extremely rugged and can handle the time. Only 250 units will be made and once they're gone, they're gone. Price is 940 euros for 1 whole watt of SET power. Check out the 6Moons review, visit their site to purchase and also check out the ZVex iMP amp also with 1 watt of power using, I'm told, the newest tubes ever made.