I've been looking around lately for a good p-bass and I just came across a 2004 MIA for a really good price. I'm no expert on P's and their features and this one has a volume knob with a button in the center that you can push in. When engaged it seems to cut the lows somewhat but certainly doesn't sound better to me, actually makes it hollower and empty sounding. What is the reason for this button? Why do I not see this on older and newer P's? I'm pretty sure sure this is a passive bass as I see no battery compartment. The bass sounds pretty good with the button unengaged, as least as much as I can tell in a crowded music store. If I buy this bass is it possible I might have regrets down the road for any reason and feel the need to change the pot, knob and or pickup? I realize that question is very subjective so I guess what I'm asking is what are peoples experience with this setup? Why did Fender do it cause it sure adds nothing to the tone for me.