Just about possible, even if you mean continuous clean(ish) power.
If you run the tubes well beyond their design limits and accept a fair amount of crossover distortion, you could get there or very close. Some Marshalls do put out a real (though not hi-fi) 70W from a pair of EL34s, and Fender achieved similar power from 6L6s in Ultra-Linear configuration.
Or, you could just use 6550s or KT88s and then it becomes very easy .
I was thinking the same thing, how can you get 80W from just two EL34?
Searching the internet I came across an article about different schematics
One of the schematics was the Super Penthode configuration.
This configuration gives you 80W from two EL34.
I asked if this was correct, but got no answer from Bogner on this.
They did gave me an answer to my Bias question from the same email.
The Super Penthode uses extra tabs from the transformer.
Maybe the reason why you can't switch to 6L6 tubes in these amps.
Thats not just extra tabs/taps on a transformer, thats a very special bi-fillar winding that has multiple specialized coils. The article goes on to say it requires true pentode tubes, and 6L6's are not true pentodes, they are beam power tetrodes.