BAT phono VK5 which tubes best to roll ?


Hello,
I am trying to find out which tubes to roll in my BAT VK5 that will generate the largest changes in sound. Normal there is a pair of 6922 that is most effective.
There are 4 pcs 6922 at the rear labelled as V101/V102 and V201/V202, 4 pcs 6922 at the center labelled as V103/V104 and V203/204.
It appears as 1 pair per channel, does this mean that both pairs should be replaced at the same time ?
Thank you for any advice.
Terry
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