From what I know, different models are best from different sources. For instance the best "Amperex" 5AR4's were from GB (they're actually rebadged Mullards). Their best 6DJ8 was from Holland. Also keep in mind that Amperex never actually made a tube called "Bugle Boy". If those words actually appear anywhere on the box or the tube itself, it's probably an eastern knock-off. Those might be fine tubes, but they're not genuine. Anyway, I don't know what source is the best for 12AU7's or 12AT7's. I can say that I've never heard anything bad about an Amperex from Britain, France, or Holland.