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?
I just put new J-J 6922s in my SF Line2 preamp except one tube is an EH 6922, and now I get some low hz thump sounds in one channel. It is very low HZ - like a bass drum beat and only starts after i play for a few songs. Also, one channel plays allot louder than the other Any ideas?
Anyone still using a SF Line 2 preamp, or are they mostly dead now and thrown away?
The Line 2 tubes have to be matched sets. I believe that your symptoms point to unmatched tubes.
Sounds like your tubes are NOT matched properly or MPs in the wrong position. I had a Line 3SE+ and it only requires MPs.

FWIW, I found Philip Holland e88cc SQ worked best ... EH and Sovtek very hard sounding.
Thanks. Do all 6 tubes need to match or just match at each stage?
Okay, so now i took out the odd ball EH6922 and one of the JJs and put back in two of the stock tubes. Most of the problems went away. The channels play at the same volume and no more drum beats. But ,now its back to making wrestling sound in one channel's tweater, which i thought was due to a week tube. I know the speaker is okay, because I switched speaker cables from side to side. I put the stock tubes in the V3 posistion. Do you know if that is the least important stage? (I am only using the RCA input and outputs too).