How do you know when to replace tubes

The only 6H30 tube rolling option I'm aware is 6H30-DR and they are expensive and tough to find plus you need 8.

Caveat Emptor buying 6H30-DR off Ebay located in Russia. I heard many received fakes.

There are rolling options on 6550. Since you only need one, you can try NOS TS, NOS GE ...

I believe your Einstein rolls 6922 so you got lots of options.

My experience is similar to Atmasphere, SQ gets softer as tube ages that will signal time to replace.

According to Kalvin @ARC, all power tubes (KT120 and 6550) regardless of position should be replaced @2000 and 6h30 @4000 hours in my amp so probably same as your pre.

I'm not taking any chances and replace all at the recommended intervals.

When it comes to power tubes, I'm more concerned with catastrophic tube failure than SQ. Following the recommended tube maintenance schedule is dependent on the amp and manufacturer.

I've had power amps where the tube just gracefully dies but with ARC, I'm definitely following their recommendations.