It adds a whole other dimension and is hard to imagine the pedal without. absolutely blown away by the sound especially with the Bias knob.

Well now I have tried it and it's AWESOME!! The Bias dials in everything from creamy to nasty!! There's a huge variance that I'll have to find my 'sweet spots' depending on which amp and which guitar I am using.BrianDear BK,Gday there BK, just got the tube driver in my hot lil hands this morning. It does not change the EQ directly but because the way the tube responds can be varied you will hear EQ differences at various settings.Most players find a 'sweet spot' about 2:30 - 3 o'clock on the control.All in all, it is a 'pro tool' type of control that each player can set to their own subjective taste.User Comments regarding the Bias mod upgrade:Hi BK,I got the Tube Driver with Bias last week and was too busy to test it out. The Bias control also adds dynamics and allows you to custom match your Tube Driver for any particular pickup/amp combination.Technically it varies the electrical 'strength' of the tube and that affects gain, texture and dynamics. Just go up or down a few clicks and he nails what he wants. Eric Johnson used to spend a lot of time looking for perfect tubes to use in his standard Tube Drivers, but now he can just set the bias easily with the stock tube. Very versatile.The Bias control is very helpful as it adjusts the amount of current applied to the cathode of the tube. It has the special Bias knob modification done by Butler himself. It is autographed by BK Butler himself with an engraving on the back. This is an excellent pedal in great shape. Pedal Auction Description: Up for Auction is an authentic BK Butler Tube Driver Special 5 knob Bias Mod.

Bk butler tube driver 4 knob