replacing tubes on amp.

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.