That's certainly overkill for this application. This amp stays cool without the top on. I can't even feel heat emanating from the any components unless I touch the heatsinks directly. I don't need convection here, just need to make it a bit more open air. Think Magni, not Asgard.

If you don't need convection, then why drill holes?  Even w/o holes you are getting convection which drives the heat transfer of a heat sink.  The question is how to best maximize the effect.  Or if it's not that big a deal and you just want o play around go for it.  Just drilling a few holes on top will likely make it worse as Armaegis said.

The question of loss of surface area w/ holes is a good one w/o proper design as a heat sink.  Size is largely irrelevant to the effects of convection, it's a matter of temperature and airflow, not size.  If you take the chassis apart and mount it on a board open face, you'll get much better convection.  Likewise, if you employ strategic holes along with fins that already have lots of surface area, the loss from holes will be minimal compared to the gain in air movement.  If you really want to get it right, one has to design the chassis as a proper convection/transfer system.  People forget that even though it is a gas, for aerodynamic purposes, air behaves as a fluid so fluidynamics are just as relevant.  Just popping a few holes in the top is akin to pouring out a 1 gallon jug of water without opening a second pressure relief valve.

http://www.youtube.com/watch?v=aSCMbZ-UAcY

Hot air creates a low pressure environment (essentially like a pseudo vacuum that wants to filled/replaced), but it does very little work without the higher pressure of cool air driving to take its place and fill the void.  It's just like weather, except the chassis is acting like a mountain range separating the climes from interacting.  That's how you get stagnation and potential for worse performance than no holes at all.

   

How hot are we actually talking about? How long can you keep a finger on the heatsinks when you've just opened the top?

In my opinion the vent thingy if done correctly (you need an intake as well) may not help that much for lowering the temps on the transistors as it may help other parts to not get hot. That is - air will slowly flow to the exit holes and not spread around the whole chassis.

But as I said - what temps are we actually talking about? Anything below 50-60°C is okayish in my book.

Exactly, to be done right you need a well thought out design for the whole system.  Always think about the system as a whole and everything in it relative to each other.

As I've said - north of 60 degrees Celsius is where you should worry. I think the holes won't change much thermodynamically, yet on the long run you might run into a dust problem.

That makes sense.

I could try to work around and drill some intake holes in the bottom but since it's already in a completed state it would be a PITA. Since RudeWolf mentioned it, I think the dust is more likely to limit this amp's life than heat.  I'd leave the top off if it wasn't for the exposed mains wiring TBH.

Changing subjects, what would be the easiest way to get my hands on an acrylic top plate?