Pentode screens are delicate (more so than beam tretrode screens) and heat up much faster than the plates. Screen dissipation increases strongly when the plates are driven into saturation. Screen dissipation is further increased with a more horizontal loadline, i.e. high load impedance. This is the reason why I warn against doubling speaker impedance.Fully dimed through an attenuator, I have seen the screens actually sag & then melt/ short out.
If power tubes actually die early when using a power attenuator, there are only 2 possible explanations. Either the tubes would have died just as well when using speakers at prolonged periods of tube saturation. Or the attenuator impedance does not match the speaker impedance over the whole frequency range.
I have seen and measured the frequency responses of several "reactive" power loads/attenuators that showed way too high impedances around the resonance and above 1kHz. The reason was that they didn't provide appropriate damping for the inductors used. This could be an explanation for screen failures.