I don't think it's possible without some modifications. The voltage requirements between 6L6's and 6V6's is quite different.

Another option you've got is to use some Groove Tube Yellow Jackets. These allow you to use EL84's in place of 6L6's or EL34's, and will lower the gain to around 18 watts.

And yet another option you've got, is to use a power attenuator such as a THD Hotplait, or a Weber Mass.