Possibly because the tube data sheets are rated at 6.3V and the brick is only putting out 6V. Based on ohms law, if you did have a tube rated for 6.3V @600mA, that means the internal resistance is 10.5 ohms. Based on that, if we apply 6V to the same 10.5 ohm heater, it would draw 571.43 mA. That is still 71.4mA higher than rated power brick, so maybe not. And I know the Vali 2+ or Vali 2 is running DC heaters, because that would mean even less voltage on the heaters, and less amps as well. So good question.