That's a serious case. It would take quite something to break it. Not like mine...* Though if I were you, I'd drill holes into the fins of the heat sink, from top to bottom. Because heat goes up, and placing the fins horizontally like you did is not very efficient.
*Yesterday I tried to clean the flux from the case with alcohol. Some solvent got into a hole with a screw and it cracked the plastic. I learned that PMMA will crack that way if solvent get on a part that is under stress.
You can see the cracks in the plastic right of the tubes. So yeah, building an enclosure out of PMMA is not a good idea. I don't know what I'll do at this point. It's cracked so badly that there's no way I can tighten the screw without shattering the plastic into pieces.
These are the new NOS 12AU7 tubes I received yesterday. I'm still testing them. The gain is indeed lower than the AX, but not as dramatically lower as I expected. I added 100K resistors before the 100K pot, something I omitted originally. At least now the pot has an useful range. I tried a few other pots, and also cleaned them, but I still get pot scratching noise. Though the scratching noise is only audible when the pot is turned to max, which is just too loud anyways.
I still have a low hissing noise in the background, but low enough to be acceptable. I have two new parasitic noises though. One seems to be a ground loop hum. I don't see why I get such a hum though since I took extreme cares with the grounds and used only two star grounds : one for the PSU and one for the amplifier. I guess I'll have to experiment some more, but I honestly don't know what to try next. I connected the 0v/ground to earth using a 100 ohms resistor. Any opinions on this? Should I just connect them directly together, or use a 10 ohms resistor?
The second new noise is interesting, to say the least. I can hear my laptop and my router talking to each others! That's something I really had not expected. I'm blaming it on the plastic enclosure, which cannot shield the amplifier from EMI. I'd really like some clarification here. Anyone else experienced such problem? Was your enclosure metal or something else? I'm starting to regret building an enclosure out of plastic. But then some people used wood... so the problem could be somewhere else.