Voltage regulators and input impedance

Why don't you fire off an email to Audio Research - their customer service is exemplary.

Also check with Modwright - it may be very easy for a local tech to increase the input impedance of your power amp to 20k without impacting the amp. If this is possible the overall sound may be substantially improved through better matching.

A loading impedance of 15K will not damage the preamp or its tubes. All it will do is impact the lower frequencies and you will not get optimal performance from the preamp. My guess is that AR uses a low value coupling cap on the output for sonic reasons and is using the 20K minimum input impedance of the connected amp to make up for it.

Tubes going bad could be a symptom of high heater voltage. Check your wall voltage. If it's greater than 120V it could be a problem for tube gear, especially if their transformer specs are based on a 115 volt primary.

If the 6550 is acting as parti of a voltage regulator circuit in the PS, the slight impedance mismatch you cite would not stress it. More importantly, what is the input Z of the Modwright at 1kHz? There’s not much music at 50 Hz. Historically, AR were known for driving power tubes hard, near their limits, and power tube failure was a common problem, but I don’t know if that’s still true, and anyway the 6550 that’s failing is not a power tube, if I understand you correctly.

Excess heat will shorten he life of any electronics, including tubes. I easily get 2,000 hours on the 6550 in my Ref 5SE, and the tubes are still working when I replace it out of caution.