Pre-Amplifier Question
You are not being very clear here. Do you want a preamp circuit or a equalizer circuit? What is your load?


C1 generally appears in these circuits as a bypass capacitor, not some sort of bass control capacitor. You generally don't want it to attenuate frequencies you want to reproduce.
First pick which frequencies you want and which ones you want to attenuate, and by how much. Then figure out the order of the filter you need. Assuming you just want some half-assed bass attenuation... you can change your filter's cutoff frequency down by increasing R1. My first instinct would be to try a 30k pot in series with that 620R resistor and see what the bode plot yields you. Be aware that you can't make the attenuation steeper with that circuit - for that you need a higher order filter.
Yes. It should be trivial to reduce the gain in circuits involving op-amps. The easiest way is to use a voltage divider at the input.