The following is taken from a leaflet entitled 'Mullard world series valves for audio equipment' it probably dates from the early 1960s.
When operated in a single-ended output stage, the EL84 can deliver an output of up to 5.7 Watts at 10% total harmonic distortion.
Two EL84's in pentode push-pull yield an output of up to 17 W at 4% distortion. As these figures suggest, this valve makes available the higher peak powers and low distortion required for present day receivers and medium power amplifiers for home listening.
The above page is taken from Mullard Outlook, February, 1954.