larbiere, I spoke to Rich at the Mesa factory repair facility about the really loud hum and high noise floor in my Blue Angel. His explaination was that the earlier amps had a noisey effects loop problem that was corrected in a later version (no serial number cutoff was mentioned). He said there were two ways to correct the problem. First is an expensive and complex mod to the effects loop that they could do on site or send the schematics to an authorized repair tech. Second was to replace the whole circuit board which was cheaper but at the time he didn't know if they had a board left in stock.
Later, after I was informed that they indeed had a board in stock, I sent my Blue Angel (serial number 005) chassis in for the board. The result is impressive. I've lost about 80% of the hum and noise that kept me from really enjoying the amp. Price for the board was $100 not including labor, shipping and some tubes I got.
Highly recommended mod if you've got a low serial number and a high noise floor in your Blue Angel.