If you have a BOLDER Modded unit, there is no tube.
The tube was only there for the A/D section. All of that section is removed during the mod. The rotary controls on the front panel do not function either. The sample rate button does work, but I prefer to leave its et on ext. sync. The 48/96 will NOT work with a 44.1 signal from a CD.
What is left is a very upgraded power supply section, the digital board, the audio op-amp and associated circuits and the I/O board. The op-amp in mounted in a socket. If you can find some different quad op-amps, you can try changing them out. I have tried a few and have settled on the LT-1362 as the best sounding to my ears.
I have left my Mensa on for a couple of years now with no trouble. Of the many hundred I have modded, not one has been damaged by a power surge.
To remove the cover, first disconnect the DI/O from power. Then remove the four screws on the sides. You can then slide the cover forward or backward and grab one of the edges. You can then pull that side out, up and off.