WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Be forewarned I'm not a tube expert by any means. Much more at home with solid state.
I'm not sure that simply changing the tube would do it. Maybe but I'd probably reduce the size of the resistor in the plate. It would be a good idea to do some SPICE sims on this to see the effects as well because you may also limit the O/P voltage swing if you're not careful.
25dB is nearly 18V/V gain and I too have no idea why anyone would want it so high. His (Michael Elliot) SA0-100 power amp is the same way. Too much gain and the driver to the MOSFETs is running a bit current starved. This is something else I would want to ook into when I get time.