Having owned a Stereo-35 years ago in college, a PAS-3X and Stereo 150 - I always assumed that it was best to put a fan on my heatsinks of the transistorized gear - to allow it to run cooler. I never removed the cage of the ST-35 - and it worked perfectly for 20-25 years before I finally sold it to a friend who was always begging for the great sound of that amp.
Recently, I bought a Stereo 70 kit from Bob and it works great - but I wanted a "classic" look so I bought an old Dynakit cage on eBay for $40 and installed it. Now it looks really perfect - and is even brown - not one of the newer black ones.
Man, does it get warm (OK, hot) on the sides and top near the output tubes. Does anyone know if tubes have a shorter life if they are in an enclosed cage rather than free? And then - what if you put a fan on tubes - would that extend their lives even more??? I mean Dyna sold 1000's of these amps for years with the cages on - so they must not be a horrible thing? On the other hand, my college roommate owned a Harmon-Kardon Citation tube amp and these were exposed from the factory...only decorative caps placed over the transformers.