Should matching pair vacuum tubes necessarily show the same brightness? They seem to sound right but one is noticeably dimmer, visually.?

No.

The brightness of the heater will vary from one tube to another

depending on the thickness of the coating of insulation on the heater wire

and whether or not the getter flash* blocks it visually.

It makes no difference because if a particular tube didn't get hot enough,

it wouldn't be capable of matching a "brighter" one.

>> A "matched pair" is often a waste of money.

Only if the manufacturer of the amplifier

states that using matched tubes is Required

should you be bothered about it.

* The "getter" aids in reducing oxygen within the tube.

A bit of magnesium is held, often in a wire loop, inside the tube.

During manufacture, as the vacuum is being drawn to evacuate the air,

voltage is applied so that the getter will do its job.

In the process, the magnesium is blasted toward the glass,

making the familiar mirror-like spot inside the tube.

Hi so without a valve tester this is not a good idea. thermionic valves like transistor needs a tester which gives the readings according to a chart. as the cathodes can be doped with more current carriers this is why some glow better than others. also not many understand that thermionic valves age and become depleted of the essential atoms.

THOSE ELECTRONIC TUBES HAVE THIN LAYER OF SILVER SHINNING COATING COVER INSIDE THE GLASS AND THIS LAYER IN UNEVEN IN EACH TUBE, THAT BLOCKING THE FILAMENT LIGHT WHILE LOOKING FROM OUTSIDE. THEIR FILEMENT OPERATE BRIGHTNESS IS THE SAME IF BOTH TUBES CARRY THE SAME PARTS NUMBER.

You DON'T match tube BY EAR.