Quote Originally Posted by DrNomis_44 View Post
...but essentially the power valve is biased so that it is operating within safe limits so it's not overheating too much.
To be honest I hadn't even thought about the amp itself overheating (something new to worry about ). I was concerned about the heat generated by the amp damaging the speaker (valves practically sitting right next to it) or causing glue to melt and the leather (or faux leather) to separate off.

Quote Originally Posted by DrNomis_44 View Post
...have a look at some pics of Vox AC30 amps, not only do they have less ventilation than my Marshall amp, they do tend to run pretty hot, and the amp chassis is enclosed inside the cabinet too.
Perhaps its less ventilation than your Marshall but it does have a vent on top, directly over the valves where the Kustom currently doesn't. I think I may have to take a saw to the top of my cab.