Good morning and great to hear from you again - do not know exact as Quad keep such details to themselves, but as there only three countries in the world (?) making them these days, then Czech Republic, Russia or China seems like a very safe guess and as they have them branded as 'Quad' then it could be any one of those? (I have heard stories about Korean and Taiwanese valves but never actually seen any that I know about.)
Also, I know for sure that at least one maker has manufacturing facilities in other countries, so even if I knew who the maker was, then I would still have to guess the country.
また、少なくともメーカーは国外に製造施設を持っていることは確かですので、メーカーはわかってもどこの国で作られているかは推測に過ぎません。
それから、このうち少なくとも1つのメーカーは国外に生産拠点を持っているはずです。ですからメーカー名がわかったとしても、国は推定するしかありません。