keeping amps cool?

Better off blowing cool air in from below into the body of the amp than at the fins.
The parts which you most want to keep cool are the capacitors.
The fins are sort of the least in need of added cooling.

I would only aim a fan at the cooling fins, unless I was certain the amp did not have a built in self bias scheme.
The Parasound HCA-3500, for instance, heats up appreciably until it reaches a set temperature.
We took the top cover off my friend's HCA-3500, to see if it would run cooler. It biased up even higher, with the cover off..

You will skew the setting even on a fixed bias amp.

Actually you're better off using the fan to draw heat from the amp instead of blowing air directly on it.

"Actually you're better off using the fan to draw heat from the amp instead of blowing air directly on it."

yes, I use a PQ exhaust fan...
http://www.hometheatercooling.com/purchase.html