After the valves, you might want to try changing the big Electrolytic Capacitors in the power supply (they smooth out the H.T Voltage). Looking at the Schematic here
http://www.schematicheaven.com/bargainb ... _mkIII.pdf
it looks like they're 50uf, you could change them for 47uf (dunno what the Voltage is, you'd have to measure it on the existing ones). It's worth replacing them on an old amp anyroad as they dry out and could blow up if the amp hasn't been used for a while.
If you're not too confident doing it (or can't follow my rubbish directions), you might want to take it to a tech to do it, it's gonna be upwards of 400 volts around there.