The two tubes on the left side need to be a matched pair,and the pair on the right need to be matched.The left side matched pair doesn't have to be the same as the right pair.The left half of the amp is biased up separately from the right side pair.Like Viridian says you don't have to buy 4 matching tubes.It can be two separate matched pairs.The pair on the left could be a different brand than the pair on the right.The left will sound different than the right if you did this,but it would work.The manual link may help.
