Thanks for your replies. I will let you know of their diagnosis as soon as I get it back, hopefully this week.
As a side note, I have decided to remove the entire hardboard backing of my TV cabinet, in the hope that my equipment is better ventilated (this had some impact on the lateral stability of the unit but should still hold – rated for 100kg, with TV+amps being ~92kg but with the latter sitting at the bottom). I have felt the top surfaces of both the pre/pro and amp several times and they always felt only moderately warm, with the amp being warmer. When I was placing them on the their shelves, on top of each other, I decided to give more clearance to the amp (~5") and much less to the pre/pro (2.2"). The sides of both units have only 0.6" clearance from the furniture. I have noticed that the MM8077 has a vent on one side, so I decided to shift it more towards the other side. Anyway, my plan was to measure the temperature of the air above the units, but I was considering installing a fan at the back of the cabinet to improve the air circulation. Do you think it is necessary? If I decided to upgrade to the AV88xx model, would it run significantly hotter than the AV7701? Would I then need to have fans in place?