It would be my guess that you could use non-matched tubes for the inputs and outputs simply because you can control the gain.

If the input tube were say a 6922 that measured 6000 M/hos on one side and 3500 on the other, if you could hear any difference in volume between the channels, which is doubtful, you could adjust the voltage output of the lower channel.

The same principle would apply to the output tubes. If you had a 6922 that read 4000 M/hos on one side and 6000 on the other, you could adjust the output if you heard a difference.

If you feel more comfortable with everything matched and balanced you could order from Jim McShane, who will do a good job for you. You could use a Genalex Gold Lion in the front position and some 6N1P-EV or Electro Harmonix 6922 for the drivers. I have found that you need strong beefy tubes in the driver position because of the SRPP circuit. It can make some of the lesser tubes crackle (arc)on startup (ex. JJ 6922; RCA 6BQ7A, Sylvania 6DJ8).  

In theory,

6DJ8<6922<7308<CCa

By "lesser" tube, are you saying that I should only use 6922s and above in the driver position?
Or are you referring to emissions strength?

I'm currently using 6N1P-EVs in all positions.

I have a mixed bunch of various NOS and used 6DJ8s, 6922s, 7308s coming in from various online orders and Ebay wins.

If I understand u correctly, I'll be sure to avoid putting any 6DJ8s into the 2 driver positions.

I just wanted to share the results of a crazy audio experiment I conducted.

I was using my CSP2+ to burn in 3 NOS tubes in the 6DJ8 family of tubes.

In the input section, I had a French Made Philips E188CC Grey Shields. On the Left driver section I had a Grey Shields Siemens CCa and on the right driver section I had a Valvo PCC88 D-Getter Grey Shields.

I plugged in my set of cans whilst the tubes had burned in for about 12 hours. I expected the worst since these were 3 different tube types and of different makes.

I was surprised that the audio tonality was superb. Classical Cellos and violins had a very deep tonal resonance. Everything sounded very natural!

Of course, there is a drawback. Due to the different tube types used for the left and right channels, instrument separation was not as good as using 2 matched tubes of the same make.