Hi there;

The amplifier you are referring to is I believe called an AM 8/4 - it had 2 large Mullard output valves in it if memory serves and was enclosed in a Hammerite painted cage affair I think.

I only saw one once in my time at the BBC - they were rare then - many many were scrapped - they were at one time the standard monitoring amplifier for driving the speakers in VT cubicles and monitoring rooms.