A small amplifier with its own speaker, plenty of gain and a high input impedance is a very useful thing to have in the workshop. A valve amplifier can be a rewarding project in its own right anyway, and can fulfil the above criteria, so this is a double win!
I'm guessing the purpose of the BNC connector is so you can wire the amplifier in parallel with an oscilloscope, so you can see
and hear exactly what is going on in a circuit?