upgrade tube for SF line 2

I am new in tube amp, I just pickup a used SF line 2 a month ago. They use soviet 6922, manufacture @ 03/2000. I heard changing the tube will make a big different. Can anyone recommend some brand and model? I do some research and find siemens CCa is a of the best. however, it is hard to find and the price is high. Some body told me I can just change the two tubes in the back and leave the front four as is. that can save some money. Should I change all six to some good NOS tube (but not the best) or should I put all the money in the two that is in the back? If I change just two in the back. In the further will I get benefic by changing another four? One last question, how about bias? Do I need to re-bias them?
What is an MP?
I think the Line 3 has more tubes than the Line 2.
The line 2 has 6 tubes total.
On the Line 3, the MPs are:

LV1 V2
LV2 V1 (6U8A)
LV3 V4
LV4 V3
LV5 V6
LV6 V5

The most important 6922 position is next to the 6U8As, LV1 V2 where most use their best MP. From my experience, the more you use, the better it sounds.

I'm not familiar with the Line 2 but my GUESS is
LV1 V1
LV2 V2
LV3 V3

You could always beam Chris Johnson an email to confirm.
There's been a lot discussed in these forums regarding the critical matched pair locations in the Line 2, if I remember correctly, the LV2-V2 set are the most critical followed by LV1-V1 and LV3-V3 being the least critical. Each pair of sockets (LV-V) must use matched tubes but all (6) pairs don't have to be matched or use the same brand of tubes, so you can experiment with different brands for each socket pair. I hope this helps.