Unless a tube catastrophically goes bad, and you will know it when it happens, it's "normally" a somewhat slow process; the amp will begin to sound "sluggish" with a sense of loss of dynamics and general "oomph." Bass becomes less defined, with more boom than a pure note with texture. Depth of stage collapses as microdynamics and ambience are lost. Your words indicate that you care enough about this and my sense is that you will notice these things and hence will know when it's time. Power tubes go long before preamp tubes; rarely do I replace the preamp tubes due to loss of performance.
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