The real deal B.K. Butlers are much warmer and have more bass response than any of the ones modeled after it. That includes TubeWorks, Blue Tube, Chandlier, the black ones made in Taiwan, etc...
He now makes them with bias knob to increase the full range of tube drive from low subtle smooth creamy gain to raging tube breakup distortion.
Edit: well maybe not the Blue Tube. It has plenty of bass but not a lot of gain.
Yeah, well, he went from company to company.
It's a shame that he has not figured out how to use a voltage pump yet