Voltage regulators and input impedance

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.

At either 15kOhms or 20 kOhms your current is extremely small. Take a 1Vrms signal your current is only 0.05 or 0.07 milliamps respectively.

Trust me, your amp’s input impedance is not causing your tubes to burn out because of maybe 20 microamps more load.  It is much more likely however to soften the high frequency response a little.

Perhaps there’s a part failure or a bad solder joint in the VR circuit that results in excessive plate dissipation ( too much wattage) across the 6550. You need to have it examined by a competent tech or by ARC directly.

My guess is the impedance issue is a red herring so far as the VR tube failures are concerned. I think you wrote above that ARC specifies an input impedance of not less than 20 K ohms. This suggests that the output impedance of the pre-amplifier is around 200 ohms. (I didn’t look it up.) An actual 15 K input impedance, even in the low bass frequencies, is not going to over stress the preamplifier in any way. And anyway the input Z of the Modwright rises to 23K ohms at 1 kHz which sneaks in over the lower limit.