Thanks - I should've said I've already seen the datasheet. What I was looking for was an opinion about their ability to operate at specifications 'outside' those printed on the data sheet, when substituted for 6V6G.
For example:

1) if I wanted to run them at 410 V on the anode, would they be alright without inter-electrode arcing and other problems?

2) the -12V grid bias seems to be on the hot side for a 350V anode voltage (compared to about -20V to -30V for a typical 6V6G). Will the grid handle a higher negative voltage?

3) the load resistance at the spec on the datasheet is 5k. Will they operate alright at 6k6 or 8k?

Cheers