Curious as to the answer to your question. I'm having problems with the blower on my 79sd. Replaced the amp but still having problems. There was evidence that the original amp was getting hot but I don't know if it was normal. When I get mine working I can shoot the amp with a non-contact thermometer to see what I get.
My intuition is that the amp generally runs warm.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13