replacing tubes on amp.

hi guys,i need some advice on changing tubes. sine my BAT amp have 4 pairs of power tubes and 8 pairs of the little 6sn7.. the question is hard to find 8 pairs of 6sn7 of the same kind in the used i need to keep the same brand on all of the tubes? and can i switch 1 or 2 pairs at a time or do i have to switch everything at once?.would it improve the sound if i replace only 1 pair of tube at a time?thanks
I would replace all the 6SN7 tubes with the brand that provided the best performance and keep the old ones as emergency back up.

You did not mention the model of your BAT, but posting that info may allow another member to suggest something proven for that circuit.

Strictly as a guess, I would use RCA black plate 6SN7, Sylvania Mil spec or (best of all) RCA 5692 red base. I would get same tubes, ( all RCA, all Sylvania , all Tung Sol, etc). This is not a difficult tube to find, there is no reason to compromise.

I would not choose the current Russian 6SN7. It is a poor example of current production.
I'm guessing this is a vk60 or vk75. The front two 6sn7's are the most critical. If you are strapped for a little cash, then replace these two first. They also need to be very low noise, preamp grade types of tubes.

Replacing all the tubes makes a diff, just those two upfront the most.

Albertporter has good advice above to.
do i need to replace all tubes at the same time? can i replace 1 pair at a time?
Albert's advice is a good safe bet for very good performance.

There are other audiophiles though that do get good results mix and matching different 6sn7 tubes; if you want to take the time to do a lot of "tube rolling". The "mixed" crowd believe using all the same tube seems to accentuate the particular tube's weaknesses.

A popular alternative is to use the older VT231/6SN7s as the voltage amplifiers, and the 6SN7-GTA/GTB as driver tubes. They feel they are playing on the strengths of both types.

I don't disagree with Sugarbrie, especially if you have the funds and patience to own multiple sets of this tube type. There are many great versions of the 6SN7 and performance likely varies from circuit to circuit.

My main concern was that "Ttrhp" not purchase premium priced Russian or Chinese 6SN7 tubes when old stocks of great tubes are plentiful.

If tests of mixed 6SN7 type tubes are done with the BAT, It would be nice for the results to be posted so all may benefit.