Mixing new and NOS tubes OK?

As long as the tubes, are electrically identical,(same family) there is no reason, that mixing, would not work. The circuit only sees, the electrical charateristics, of the tubes, not who's name, is printed on them.

This is becoming a practice in AI preamps, though they only use 4 6922's (two in the phono and two on the line). I set a friend up (who has a Mod3a) with old production Russian 6922's (from ATSI which are better than the recent Sovteks) and pair of NOS Siemens E88CC's (which failed within 4 months). Now I have a pair of NOS Siemens 7dj8's for him to try with the Russian tubes. We will see how this goes, though people at AA have had good results with this combo and it looks promising. I do not know what the duty of your (8) 6922's are and would suggest talking to BAT about it.

This approach works in the SF Line series. My Line 1 has a mix of 6H23s (2 pair) and 7308s (1 pair). Don't know how it works in the BAT gear.

BTW, the 6H23s (aka 6N23) are from Upscale Audio. It's an especially nice tube for the money.