OK listen close...Once you have removed the 2 screws on the sides and the 2 small screws on the back...Your there! Pull the dam lid towards the back of the unit. The reason its so hard to pull off is because they put glue or adhesive or something to make it sticky. Just use more force I promise you wont hurt anything, there is no big metal lip holding anything just pull it back . You do not have to touch the front faceplate or End caps. You'll get it, just pull it harder it'll come off. As far as tubes are concerned I would replace them all with NOS Mullards (british made) the 12ax7's might cost you $140 a piece but the NOS 12AT7 Mullards are running around $40 and you can get them from Kevin at Upscale audio.
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The C220 uses only 12AX7's, 2 for the line stage, and 2 for the phono stage. The C2200 uses 12AX7 and 12AT7. I think Matt's recommendation for Mullard is good, although there is many different ones, depending on vintage and some made for military. Have a few friends that like Telefunken, also some variety, in their MAC gear. Here is a link with lots of information on 12AX7's from another reputable dealer.