i'm a big fan of running fans. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
seriously, any external heat sink will benefit from a fan, placed to blow across the heat sink. The direction of flow is also important as the opposite end will be hotter than the side closest to the fan. think about what is on each end.
keeping the amp cooler, i.e. helping convection work, will not only prolong the life of the amp, but also give you a bit more thermal headroom.
there is no real hard and fast rule as some amps have good thermal design, and some just have thermal cosmetics.
the placement of the fans is also key. note that fans will pull dirt and dust into the amp. what you want is fans on the heat sinks.
that's why i'm a fan of xtant's cooling design. the fans don't get the board dirty, they just cool the heat sink fins. granted, the earlier versions were flawed because they left some heat producing components off the large heat sink - and they fail. doh!
