Electronic – What type of glue would you use on set potentiometer screws on a PCB

You could use a potentiometer (pot) for this, but they are typically continuous in their output where it sounds like what you want is discrete values. To use a pot you would need to use it in a voltage divider and set bins (voltage ranges) for the various values you want to set to, and read in the output of the divider with an ADC. To set this you would adjust the pot until value of the output falls in to one of the bins. This person made a pretty neat feedback system for this idea. You can also buy pots with detents, detents are mechanically set positions through the turn.

Alternatively you can use a rotary encoder, they're those knobs that spin forever in either direction and "click" in to set positions along the way, also by use of a detent. There is a wide variation in encoders. I've recently used these ones. Basically you can continue turning these around and around and they send pulses to your microcontroller letting it know which way they are turning (this is a slide set explaining how they work). By counting these turns in the respective directions your microcontroller can set values. For instance turning right increments the value while left decrements. This is nice when you want to use one knob to set variable ranges, a solution you can drop in to most projects that need a knob.

Normally reactive components such as inductors and capacitors are used to provide attenuation within/outside of specific frequency ranges and this by definition is how cross-overs are built.

Reactive components are favorable because they do not dissipate power apart from minimal R losses.

If you want to attenuate the signal being driven into a power load such as a speaker then I believe you will need a fairly robust potentiometer. If you are delivering 40 watts into a speaker, and you want to attenuate it by -3dB, then your potentiometer will need to be 8ohms and capable of handling 20 watts. Something the size of a high school lab rheostat would be suitable, but impractical and slightly inductive. I would suggest you consider attenuation of the signal entering your amplifier rather than the signal coming out of it.

If however you're dealing with 250mW headphone speakers then the potentiometer will work, however you may want to consider a Logarithmic type as the parallel impedance equation is not linear with differences in R.