Finally plucked up courage to stick my lovely possibly NOS quad of yellow-print Mullard EL84s into the TL12 Plus. They sound stunning, the missing piece. The Russian 6P-14P-ER I usually use are fabulous valves, stunning value, but just a little bright in the TL12 Plus amps. They suit the Stereo 20 better. The Stereo 20 isn’t really 2xTL12 Plus even though that was the design brief. The input valves (ECC83 vs EF86), phase splitters (ECC83 vs ECC81), and the output transformers are all different, and these amps do certainly sound a little different. The TL12 Plus a little more upfront and forceful. The Mullards really suit them; wide open, plenty of dynamics, bite and speed, but no harshness. I may as well use just them here for a while as my hearing won’t get any better than it is now with increasing age, and they aren’t something I’d ever sell. So a full vintage Mullard set right the way through!