Digital gear with tubes in the circuit should simply be left
turned on (as should digital gear without tubes)."
Raquel (Threads | Answers)
Alex Peychev, designer of APL HiFi tubed digital sources, suggests otherwise
regarding his products. In fact, he recommends turning the APL units off at
night. I believe this is because of the wear and tear on the electronic circuitry
due to heat rather than wear and tear on the ECC99 tube.
Thus, my recommendation of seeking the advice provided in a component's
operating manual, or seeking the advice of the designer.

