just tried a BK Butler with bias control on guitar last night.
that is one nice bit of kit.
as Musky says - the bias control is a great attraction; but it's not all that's so nice about it - it produces distinct and delicous layers of creamy distortion that cheaper pedals usually don't get close to IME
I compared the Butler to a Keely Tube Screamer, an HBE Power Screamer and a Catalinbread SFT
It smoked the two Screamers in no time, but then they are different animals.
The SFT gave it a serious run for its considerable money though.
The SFT could produce the same distinct layers of tube amp like distortion as the Butler, but doesn't have the bias control.
I think the Butler's layers of dirt were slightly more pleasing to the ear; but not much in it really.
the Butler is probably about £300 compared to about £120 for the SFT.
Pound for pound the SFT wins; but the Butler is a dream piece of kit IMO.
.....if only there was a bass available at the time of testing!
PS. the Butler produces its own natural tube compression, the SFT needs it to be added, which I did with Keely 4-knob (at £175)
So the SFT and Keely 4-knob is £300, compared to the Butler at £300....hmmmm, not so straight forward a price comparison now.
If all you wanted was incredible amp-like tube drive and distortion, then I'd get the Butler
But then that Keely is a great compressor and it would be nice to have that available on its own for use with other toys......