The two tubes in question are the 6021W. They have 2" wires coming from the base, not the rigid tube pins you see in most other tubes so they can't easily be used with tube sockets. They must be de-soldered and the replacemnents have to be soldered in place. By using a miniture tube that is soldered you gain many advantages over a standard tube/socket: Vibration has less effect on the tube, the tube is more rigidly mounted, connection noise is lower, the tube will not fall out or get damaged in shipping the unit. BOTH MK1 & MK2 use the same wired tube so user replacement is difficult. As stated in an earlier email, I have NOT seen a tube failure besides the one that had a slight hum in one channel and there are units have been playing for OVER two years!
If a tube needs to be replaced I can get it done and return the unit in 48 hours.