The settings used on any crossover can be very subjective and will vary from situation to situation. However, there is a basic starting point where anybody can begin. The following is a breakdown of the front panel controls on a CX3400. As an example, the recommended starting points for the controls are listed when running the unit in 2  WAY STEREO MODE:
1.) INPUT - This controls the amount of signal coming into the crossover. Start this off at 0.
2.) LOW CUT- This button cuts out all frequencies 25 Hz and below. Start off with this button OUT (not engaged).
3.) LOW/HIGH XOVER FREQ (The first XOVER knob) - This knobs controls the frequencies that will be sent to the SUBS and the TOPS. As a reference point, start this off at around the 120 mark. To hear more or the low frequencies in your SUBS, then turn the knob counter clockwise. This will start to cut out the low-mid frequencies sent to the SUBS, leaving more of just the SUB frequencies to be heard. Adjust this to taste.
4.)  LOW MID/ HIGH MID XOVER FREQ (The second XOVER knob) - This knob controls the mid crossover frequency control when the crossover is set to 3 WAY STEREO or 4 WAY MONO. This functionality is not available in 2 WAY STEREO MODE.
5.) LOW DELAY - This knob is used to delay the SUB frequencies sent to your speakers. This can come in handy to correct phasing issues created by distance discrepancies between the TOPS and the SUBS. As a starting point, set this knob to 0
6.) LOW OUTPUT GAIN - This knob controls the level of the LOW frequncies. Start this off at 0. Use it as a SUB volume control and adjust to taste.
7.) MID OUTPUT GAIN - This knob controls the MID output level when the crossover is set to 3 WAY STEREO or 4 WAY MONO. This functionality is not available in 2 WAY STEREO MODE.
8.) HIGH OUTPUT GAIN - This knob will be used to control the volume being sent to the TOPS. Start off with this knob set to  0 and adjust the level to taste.
9.) MUTE BUTTONS - These buttons cut out the volume of their respective GAIN knobs that they are next to.
10.) 0 INV- These buttons are used for PHASE REVERSAL to correct polarity issues that can potentially be caused by the wiring being used in the setup. These buttons can be OUT (not engaged) in the beginning. You can experiment with them to see if they help to correct any sonic phase issues you may notice in your setup.
11.) CD HORN- This button is used for HIGH frequency correction in constant-directivity horns.
12.) MULTIBAND LIMITER/ THRESHOLD - This knob sets the limiter threshold. Start it at the 0 mark. Experiment with this knob while it is engaged to see how well it will work for your situation.
Now, with all of that explained, please take note that when you have the CX3400 set to 2 WAY STEREO, green lights will appear above the knobs that will be used in that mode. You should only concern yourself with those knobs where the green lights are indicating.