I don't think you did any damage. I'm not sure though. I see it as if you used the "screens" as if they were the grid. With the grid (screen) still biased relatively negative to the cathode and b/t the cathode and plate it would seem this would not cause any harm... however not sure what happens in there electronically speaking. I'm curious, how did it sound???
For biasing those tubes use ohm's law:
Max PWD for 6v6 is around 14watts. 14/327=43ma. That is your maximum!! I think you could get away w/ as low as 125ohms for a cathode resistor, but it'd be best to try one around 200ohms and go down from there while monitoring the current. Be sure it is rated for at least 15watts.