What tubes for Mac MR-71 tuner?

Musicslug is right. I have original tubes with my MR71 but aligned and checked every three years, and it sings all the time.

The "original" condition of the MR-71 should not be used
as a basis to judge it's performance capabilities.
I recently acquired one and have replaced components that
have aged, i.e. capacitors, resistors etc.
A few tubes were also not performing correctly, they too
were replaced.
The performance after all this is done is outstanding. It's
like night and day. It's well worth the effort.
Only then can one realize why these units are legendary.

Before you go doing that, check the tubes out to see if any test bad. Tuners aren't that hard on tubes and they may be just fine. Probably the best thing you could do before you start 'tuberolling' is get the unit calibrated. That could make a huge difference in its sound if it hasn't been done in, say, a decade or two.