When your Amp Clip lights light up then it means that the Amp's Input is being overloaded which you can correct by either turning the Amps gain knobs down a bit or turn down the Source Input, the Behringer.
The way to know which should be turned down, the Amp or the Behringer is all about "Gain Structure" see below....
An occasional clip light is not terrible but I prefer to never see it at all.
You can set up your Gain Structure for your System by connecting your Source Input to the Behringer and then the Outputs of the Behringer to the Crown XLS 602's Inputs.
Do not connect your speakers yet, or if they are disconnect them.
Turn the power ON to your Source (CD player etc..) the Behringer and the Crown Power amp, leave the front Channel knobs on the Crown all the way down.
On the Behringer wether you are using Inputs 2/3 or 4/5 I would leave the Level for the channels at "0"
I would leave the "Main Mix" knob at "0" straight up also.
Now turn up your Input source to the Behringer until you just see the Red Clip LED's light up for your L&R Outputs.
If your Input source such as a CD player, does not have a volume control and instead has just a fixed output, then adjust the Channel Level knob until you see the Red Clip Led's light up at the L&R LED Meters.
Red lights should just Flicker and not be Solid on!
Now on the Crown XLS602 Power Amp turn up the front Channel 1 and Channel 2 Gain knobs until you just start to see the RED Clip lights start to flicker.
Once the Clip lights start to flicker on the Crown turn the Channel Gain knobs down just a bit until you do not see any RED Clip lights.
I would rather set the System so when the Red lights on the Behringer flicker, the Clip lights on the Amp still will not!
Now if the Behringer happens to hit Red the Crown will not and you will not Clip the Amp.
Your System's "Gain Structure" is now safely set.
Some will set up their System more on the edge to tweak every ounce of Power that they can!
They set the System Gain Structure so that when the Red Clip LED's light on the Mixer, they are also lighting at the Power amp, then they know if they see Red at the Mixer they know the Clip lights are also lighting up on on the Amp.
You then need to be quick on the Levels to pull down what is Clipping the Mixer.
I and many others take a more conservative approach that I'd rather be Safer than Sorry.
If I see an Occasional Red Clip at my Mixer, then I still know that my Amp is not Clipping and my Speakers are OK.
You can set these two whatever way you want, just so you know when the Amp's is Clipping!
My Subs are getting 2800 watts each from a CE4000 and at the higher wattage, Clipping can send a lot of heat to my voice coil and cause damage a bit quicker.
I also have more headroom and can back off on the Amp's gain a bit more to guarantee no clipping.
Clipping is one of a major cause of Speaker failures today.
The only better way to set "Gain Structure" would be to use a solid test tone but Source Material is acceptable, many put on a CD and set the Gain.
Now just watch the Behringer L&R LED's and from the method that you used to set the Gain Structure you know when the Amp is clipping.
I have also seen that some CD's are mixed hotter than others, so when you change the Input Source (different CD) make sure that the Behringer red clip lights do not light up or do so rarely.
If your System is plenty loud then leave your Mixer in the Green and then if a hotter Input is used you have some headroom.
I also always leave my Main fader or in your case Main Mix Knob at "0" Unity!
If you need to turn down the Behringer, use the Channel Level knob or turn down the Source.
At this point if your System cannot get loud enough for your taste, then you need a bigger Amp or Speakers because your System is running at or pretty close to it's max.
I've tried this, yes it really works and it's a good idea, but as you where saying some songs are more powerfull than others and those may clip your amp. So I should be cearfull.