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.
GM70s also have comparatively short service lives at the operating currents and voltages where they perform best. (In my power amps they're done at around 1200 hours of use)
Smaller DHTs like the 45 and triode connected DHP 4P1L can be quite good and are practical for a line stage or buffer application.