I don't see any troubles, the g2 is rated for 20 watts, and that's 30 mA at 666 Volts. ~700 looks like a good OP with an 8-10k a-a load. Figure g2 current at between a fifth and a sixth of the anode current and we're O-Tay....

Run anode voltage too low and the g1 bias will be small in comparison to the filament, and that can contribute to hum if you're not careful.

It is a cool looking valve, and pretty tough in the ratings. Say 600V for U-L rigging with 30% g2 and E-Linear tapping and run 100 mA of idle through the anode and I suspect you'r get a really nice 30 Watts.

I am building with its little brother the 4E27. 65 Watt plate and a 5V/7.5A filament instead of the 813's 10V/5A. Both thoriated W filaments, the '27 is a true 3-grid pentode, and the 813 is a beam-Type. Who knows which will sound better. I am sure either can be harnessed.
regards,
Douglas

Praise the Lord, and pass the 813's!