I have a 4-string Spector Euro series. I have EMGs in it, PJ set up. I love these pickups. I've heard that if you install the BTS system it will make them sound even better. (puts a blend in and a master vol. plus the treble and bass). But while looking up these pickups on the net i came across the BQS system. There is 4 knobs but 2 of the knobs are stacked. My question is... what are the 2 stacked knobs? I think one is bass and treble while the other is mid and ???. Also are these worth buying? They seem like they would allow me to get even more tones and control with my bass. Thanks for info!