I can see four large tubes through the top cover.
...I bet those get quite hot when the music get serious; and that would "brownish" the top cover (in & out), just above those tubes, no doubt about it.
I would love to see a picture showing the internals.
Have you heard how the new Zanden Choku amps from AE are? I have spoken with a few people who were recently in HK...I think they heard the Cessaro setup at AE with the Zanden Choku monos...except they did not A/B the amp against anything else. The main A/B I think was removing 1 Thor SE grounding cable from the new Emperor which was supposedly a real ear-opener for those who were there (as in, removing 1 cable really dropped performance).
Yes, I read AE forum regularly. Choku are supreme amps, no doubt about that, but 8120 is interesting because of the price and the performance isn't much compromised as said Mr. Yamada in the TAS (243) designer roundtable.