I would replace to signal tubes first, research and get the best you can afford. Why they will last. You will have to replace the power tubes because they burn out much faster. Just my experience.
Signal tube versus power tube, sound impact?
My mono block has eight KT 88 tubes(power tubes) and six smaller signal tubes. I'm using the stock tubes that came with each Mono. I think they are JJ tubes.
Rather than replace them all would it be best to change out the signal or the power tubes? Not sure how good the JJ tubes are but I hear they may not be the greatest.