I've got a Coopersonic Tube Overdrive and I am now starting to experiment with putting some NOS tubes in it. I have heard that people with Varidrives will often put 12au7's in to reduce gain and increase smoothness. Just bought a Mullard 12AT7 down at the local guitar store and replaced one of the Russian 12AX7's in the first gain stage of my Coopersonic. Reduced the gain a bit, added a bit of treble and made it somewhat smoother over all. I am thinking about getting some 12au7's and 12ay7's to experiment with. What tube pedal(s) do you have and what tubes have you tried in them?