Does it sound good? If yes, then leave the tubes alone. You may want to invest in a tube tester or find a local shop that will test them for you.
If the sound is muddy or the bass is breaking up, then you may be due for new tubes? Also, do the tubes flare when you turn on the amp (give a quick bright flash before settling down to a blue or orange glow).
If the sound is muddy or the bass is breaking up, then you may be due for new tubes? Also, do the tubes flare when you turn on the amp (give a quick bright flash before settling down to a blue or orange glow).