I know I'll be corrected on this... I am a bit confused.
But Amperex = Philips, but they built valves for Mullard and visa versa.
Amprex OG's were generally Holland. The white label and 6922's New York.
All a bit jumbled. Just look at the Philips Valve Codes in my prior post.
Now, I don't know if they compiled the list and everybody agreed on the Mfg code
It appears so because Siemens and Halske, Valo, and Telefunken are on the list.
Clearly, as an example, Philips made "French" RTC valves in Holland.
To your point:
I like Amprex valves. I paid more for the "6922 Class" valves and I believe they sound better.
I think I paid $240 for the 1960's
By far, by a mile, my "French" RTC's E188CC's sound... just magical. Can't remember. 1965 I think.
They have the Big "O" getter. I can't say enough about them. They were made in a Philips / Amperex plant in Heerlen Holland.
I have not listened to the 3 mica stuff and above, but I can't imagine anything sounding better than these.
RTC Purchase
You must find some. I paid $140 but most of them are almost $300.
I do not want to recommend the RTC / Mullards, until I hear them.
They are not the same as the RTC's I own now.
Amperex.... Early 60's Orange Globes <$ Bugle Boys $ < 6922.
I don't have any stamped "PQ" or "SQ" I believe those sound amazing and I believe they conform to the E188CC spec.
I know you have a pretty good collection.... For those starting out... IMO... pay $70 and get some mid 60's matched Orange Globes.
Those are a great value. mercedesman usually has them. I have 69 dimple OG's, I gave away 67' O getters to a friend. I had Bugle Boys and found them harsh.
IMO. It is worth paying more for "O" getter Orange Globes. They are an excellent place to start.... or stay. A nice elevator ride up from stock.