You can buy tested and reliable, early 1960s vintage Amperex 6922 PQ USA White Label tubes weekly on E-Bay for no more than $150/pair (and often less). $400/pair sounds like "D" getter tubes, or perhaps pinch waist "D" getter tubes. If not, $400 is a complete rip-off for Amperex 6922 PQ USA White Label tubes, and only the seller of those tubes will vehemently defend the price.
Be sure to complete your homework before you start buying.
Be sure to complete your homework before you start buying.