Hey Arzi,
I see there isn't much space for a 4th pot on the control panel.
http://www.pitbullguitars.com/wp-con...4-new-main.jpg
but you can get a concentric stacked knob I had to for my IB-6S.
Yes one of the controls was a blend pot.
I'd post a link of the pre-amp you are thinking of ordering and Weirdy should be able to knock up a wiring diagram