Let me explain caps for a sec. A capacitor allows high frequencies to pass through it, and lower freq are usually blocked. DC voltage, which is at 0 frequency, is always blocked. Thats why caps are used to couple two separate transistor stages, it isolates the DC from one stage so you can set up the proper voltage bias for the next one.
Now, the amount of high frequency and cut off are dependant on the value of the cap. Think of a cap like a ramp. Big, slowly moving low frequencies are going to have a hard time making it up the ramp, so they are blocked. Fast, light moving frequencies can easily pass over it. The steepness of this ramp is the farad value of the cap. Small farad values, like picofarads, are like nearly vertical ramps. Only the fastest, highest frequencies are going to make it past the ramp. As you increase the farad value, the less steep the ramp is. So lower frequencies are going to be able to make it over. As you increase you let more and more frequencies to pass, until it hits the inaudible range.
Now, that is a 'high-pass' filter. This is what the 'bass-cut' is. You cannot 'boost' anything unless you add active electronics to the guitar. You CAN, however, create a band pass filter. That is done by setting up a high pass filter followed by a low pass filter, with a narrow band of frequencies that are able to make it through both. But I'm getting ahead of myself.
A low pass filter is very much like a high-pass, except its like we set up a ramp, that high freq are going to be able to go through and slow ones can't, but we make it exit off of a cliff, so they get tossed away. What I mean is, instead of making the signal pass only through a cap, you put a cap on the signal line and make the other end of the cap go to ground. This 'tosses away' the high frequencies. This is how a normal tone control works.
String two of these together with values selected so that you pass both high and mids, then have a low-pass filter after that with a higher cap value, so those highs you let through get thrown away, leaving only the mids. This is probably not going to sound the way you want it to, though. The signal will lose a lot of its frequencies and be pretty thin and low volume. You can try it if you want, though.
Designing a mid boost is more complex, involving a band pass filter into a gain stage and recombining it with the original signal. You'd be much better off just buying a Tube Screamer and keeping the gain low, so its just a boost pedal. TS pedals are mid boosters. Or, buy a prebuilt active circuit to install, OR get into making DIY pedals. An active guitar preamp is nothing more than a small pedal circuit crammed into a guitar. I just personally don't like the idea of having to change batteries in my guitar in order to play it.
