Tube Replacement Problem - Audio Research SP9 MKiii- V1 Tube Goes Dark w/ new V2 installed

Hi, Does anyone have any suggestions??

I’ve tried replacing the original Sovtek 6922 V2 position tube with a vintage NOS 6DJ8/6922 Amperex SQ circa 1969, but the original V1 position phono tube goes dark and I get no sound from my turntable. The new tube in V2 position works fine and sounds terrific despite the no phono with the darkened Sovtek in V1.

However, when I go back to the original 6922s in V1 and V2 position everything works fine and tubes light as normal. Do the new tubes need to replaced as a matched pair or is there anything else that I should know about replacing a single tube?

The unit is a factory SP9 MKIII.
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I have owned all the SP9s and currently use a MK3 in my home theater.

The Amperex tubes are a popular choice and should work great.

Sovtek tubes are junk IMO. They are reliable but sound very cold!

Most owners of the various SP9s threw out the stock ARC tubes right off the bat, and switched to NOS.

Even the best NOS tubes can go bad, and prematurely fail.

Sounds like you have a dead NOS tube. Buy a new set of NOS tubes. There are plenty available from reliable vendors.
Thanks Don. 
If the amperex were dead then why would it be working well in my ARC LS1? It’s been in there a couple days now, glowing and sounding great. 
I did order a new set of tubes from Tube Depot for the SP9 mkiii and look forward to trying them this week. 
Soundslike half the tube is working. They are balanced valve right? I think in one position, where they work and  are wired to the side that works. In the position where it stops or goes black, just the opposite.. One half is working one half is not... That seems logical. confirm it with a tester or a schemo, ay.. either or...