It is fairly safe to run 6L6s in the EL34 bias switch position in the Spongy modes, especially Spongy/Tube which is much too cold normally. The problem is that the 6L6s will then run hot and the EL34s will run cold, which is the wrong way round for what the conventional thinking is, for getting the best from each.
It would be difficult to find two pairs of both 6L6s and EL34s that bias correctly at the same bias voltage, for all the plate voltages that the Dual Rectifier can have - which are different for the Bold, Spongy, Tube and Diode modes. 6L6s need about 10V more bias voltage than EL34s, but it changes with the plate voltage.
To modify the amp so it would work well, you would need to separate the final bias supplies and add another pair of phase inverter coupling caps so the two pairs were running independently - not that difficult, but you'll need to add a piece of tag strip or similar, as the existing board layout won't support it. The Road King must have this extra circuitry on the board as stock.
