A meter is critical for getting correct placement in your room for the smoothest spectral balance - especially important for get the bass right - but this is off thread. I think that tubes in Class A is not affected by loudness, other than clipping, and A/B is. Probably a good question to ask at Audio Asylum in the "Tubes" scetion.
A question I suspect reveals a lot of ignorance...
Does the volume at which I listen affect the rate at which my tubes "age"? I'm thinking more volume requires more voltage and higher output voltage levels represent "wear" on the power tubes. Thanks in advance for your tolerance.