• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Should I replace unknown-hour tubes from 2001 that came with an amp I bought?

The amp is an Audio Research D70 MkII. The tubes are Svetlana branded with "01" date codes. Beyond that, I have no idea how many hours are on them. Everything seems to work properly, no arcing or sonic issues.

Should I have them tested? Should I replace them with new? Should I run them until they start causing problems?
 
The tubes might last a lifetime, or they may blow up the next time you turn the power on.

"All things are equally certain, just some things are more equally certain than others". (I have to apologize to George Orwell's quote in "Animal Farm", I stole it, and modified it slightly).

Vacuum Tube amplifiers are supposed to make you relax, they are not supposed to make you loose sleep.
Enjoy Listening
 
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What are the tubes, brand, age and type? How hard are they run? There is no hard and fast rule.

If you have set the bias and it drifts quite a lot when you check later, that can be indicative that the tubes are not in the best condition.

A tube tester that measures the characteristics of the tube can help pinpoint how the tube is aging.
 
They are a mix of New Sensor Sovtek and GE. All tubes with visible date codes are maker 2001 (that’s the four power tubes, not as sure about small signal). Amplifier is Audio Research D70 MkII: arcdb.ws/model/D70:

(2 matched pairs) 6550
(1) 6550
(1) 12AX7
(3) 6DJ8 (ECC88)
(4) 6FQ7/6CG7