I have been fiddling with the valve compliment of the Leak Stereo 20. I came across two E80CC pinched waste tubes in my valve room, and checked them out on the net, turns out they can be used as a substitute for 12AU7s, as you know the leak in that first position uses a 12ax7, I had replaced that with a 12au7, so in that position only and leaving the other two mullard long plate 12ax7s, i dropped in one E80CC..... and WoW just brilliant. The question is will it cause the amp any problems, i will need to be check this over time, especially the mains transformers temperature, but for now the sound is just marvellous. Much better than any tube I have tried in that position. The model I am using is a pinched waste E80CC Special Quality made in Holland in 1957 with the Heleen made delta mark on the bottom of this tube not on the side as with later models, the Best available... they are very tall about twice the height of the normal 12au7 or 12ax7. This model pinched waste E80CC is the best out there, and not cheap, but well worth it. Why is the waste pinched on this tube, because up until around 1958 they could not make the glass sides straight. As to why they sound better, most audiophiles say, the thicker curved glass is the reason.