Is your pre-amplifier tube or solid state? If it has any tubes i recommend focusing on those as they are the beginning of the food chain.
I upgraded older tubes on a preamplifier which had two tubes to Amperex Orange pq red tip. After making this change i did not feel there was a need to change out 16 power tubes on my monos. these tubes were on the pricey side but a lot cheaper than replacing 16 Power tubes.