For further information, the Mullard and Amperex are vintage tubes. I got the Amperex upon recommendation in these forums. The etched factory and date codes are:
Mullard: B9K2 - produced in Blackburn, UK, 2nd week of November, likely 1969 (see
http://www.audiotubes.com/mullcode.htm).
Amperex #1: <Right-angled triangle> 6E3 - produced in Heerlen, Holland, 3rd week of May, likely 1966.
Amperex #2: <Right-angled triangle> 6G1 - produced in Heerlen, Holland, 1st week of July, likely 1966.
The 2 Amperex reportedly tested very well and are a great match for the ZBOX. I paid $52 for the pair some years ago. All tubes are working in the ZBOX and should have some resale value, however I prefer selling everything together.
By the way, I have 100% positive feedback on eBay. If you prefer the reassurance of working through eBay, I'll be happy to do so.