You can repeat the voltage checks to see if the tube's operating point has drifted; that's a sign that the cathode's emission is fading. Check terminals 27, 37, 32, and 42 for plate voltage. Also check terminal 9 for the heater voltage. Seduction runs a little low here, to extend the tube's life, but a low power line voltage for example might pull that voltage too low for proper operation. I would say that 6.0 volts is the target, and 5.7 volts is as low as you want to go. If you have actual 6DJ8s and the voltage is below 5.7 volts, get some 6922s - they draw a bit less current which will raise that voltage.
Do you leave the preamp on all the time? That would be nearly 9000 hours in a year, which is a decent lifetime for small-signal tubes. I would expect a really good tube to last quite a bit longer than that, but finding such a tube gets harder every year.