The 5C1 Champ ran its 6V6 at about 9 watts, the 5E1 at 11.6 watts, the 5F1 about 12.2 watts, and BF/SF Champs generally over 14 watts, so don't think there is ONE magic number you need to hit or it won't sound good. If we're voting, I'll cast another vote for R.G. Keen's bucking transformer over a Variac. A 6V/3A filament transformer is only $15 at AES, and that would be plenty to knock down your voltages into an acceptable range. Overshooting and running the filaments way too cool is also not good for business. BTW @King Fan, nice work on your Vintage Voltage box!