At www.18watt.com there are several simple gain mods for the 18W circuit. Jyrki had tried one of these: “I also modified circuitry a little, now the first triode is boosting the second stage. It still cleans up with guitar’s volume control, but does Iron Maiden at heavier end.”
This does change the dynamic response of the amp, however: “I ended up undoing the gain boost job. I’m happier with the basic grind and still can use whatever pedal needed for heavier stuff. Some things just should be left alone.
“Since then, Jyrki’s added a half-power switch and a built-in L-pad attenuator. “Oh, by the way, I did a gig with the amp 2 weeks ago. It was actually our fourth gig And it seems that it will be my main gigging amp for the foreseeable future. Managed nicely unmiked in small pub, at half power and a little bit of attenuation.”