cooling my Krell monoblocks

First, be careful aiming a fan at Any piece of gear. IF you are somehow interfering with the natural convection (hot air rising) currents that the heating of the piece causes, you could actually make it run hotter, or too hot.

Premium (read as HEAVY) power amps like the Krell should be able to take any power demands in stride. Especially when used in an open installation. The Krell 300 or 400 integrated is a possible exception. My brief exposure to one (the 400) showed it to run bigtime hot. For my confined rackspace, that was a deal breaker. The Stereophile test confirmed my impression when they remarked on the heat of the amp after the pre-conditioning period before bench measurements, Worse Case Scenario, to be sure.

How much are you asking of these amps? I don't see how you should be able to put a dent in 'em....... If they have not gone into thermal protection then there is probably nothing to worry about.

All electronic lifetimes is lowered by heat.

The only thing I know of that really likes heat, is indeed, the Cat.

I had one sponteniously combust on me after it lay in front of a fireplace for a couple hours.

Again, I urge caution.
ALL amps without fans as a designed in feature are passively cooled. The idea is that warm air rises and pulls cool air in from the bottom. Running under normal conditions in a 75f house, this should be enough. Stick kitty on top and finish the job by clogging it up with cat-hair and dust bunnies and you've got an overheating recipe.
Extreme use, like averaging about 1/3 power will stress SS to the max. Add a clog or 2 and there you go.
IF a fan is used, I don't think it should blow AT THE TOP of any gear. This will force air backwards down into the unit and restrict natural convection. Still air in the amp will just get hotter.
Many years ago I designed and built a small (4"box fan) that fit snugly on the back of my Carver Cube. It blew OUT, sucking air thru the amplifier. I could and did run it all day long just short of clipping without it cooking.
In general, I would say that pulling air up/out of an amp with internal heatsinks and pulling air over external heatsinks would be best.

Classic.
Thanks for tips on quiet fans. I may have need sooner than later.
Also, your fan use sounds correct. 'Draw the heat out'.

An annual cleaning / dust clog hunt may not be a bad idea, either.

You Guys are KILLING me.

The Krell were a mythical race in the '50s SciFi classic
'Forbidden Planet'.
Just my opin, but one of the most literate / intelligent well paced SciFi flicks ever.
A great cast including a very young Ann Francis among many others, including Leslie Nielsen in an early, serious role.

And, from the Audiophile standpoint an incredibly interesting soundtrack with all Theramin music/effects. (pre MOOG by several decades) Must hear on good system, but you already knew THAT.

Actually, Forbidden Planet was based on Shakespeares 'The Tempest'.