What do you hook up first, mixer or power amp oreffects in a rack system?

I'd need to know what mixer you have, to know if there is a [line out] connection socket and an [internal amp in] connection socket.

"each with own poweramp" I presume means the ep1500's and NOT that the speakers are active (ie containing amps)

***If any of your speakers are active, ask the question again, with more detail.***

So if all your speakers are passive, I offer the following.

I assume there is [effect send] and [effect return] on the mixer

effect send>quadraverb>effect return

mixer line out > eq > compressor > sonic maximizer > crossover

sub output of crossover> amp > sub
high output of crossover to mixer's internal amp input
mixer's speaker output > main speakers

monitor output > other side of eq (>other side of compressor if desired, but use sparingly ) > other side of sonic max if it is true stereo , otherwise skip > amp > monitors

If your mixer doesn't have an input to its internal amp, we are "back to the drawing board"
With questions like this it pays to give as much detail as possible; model and make of each component etc
I have done my best with the the info you have provided.
Don't feed amp outputs into powered speakers

You are somehow feeding back. Maybe the effects are on with an echo setting. First thing: Make sure ALL interconnected equipment are powered from the same receptacle or source. This includes amps and speakers and any other devices that are powered. Make sure ALL interconnects use BALANCED interconnect cables either TRS or XLR type. First thing is to eliminate ground loops by the above proceedure. Note the pros run power back alongside their snake from teh stage. Using multiple outlets in a building to power equipment puts equipment at risk and picks up noise due to ground bounce.

In order for this to work properly it must be inserted between the last pre-amp stage and the power amp. Can you please update this with the mixer that you are using? With the make and model of the mixer, I can provide you with a connection description. Since each mixer will have different labeling and options, it is not possible for me to accurately describe the connections without hte make and model.

Thanks,
Dan

guitar to pedal then to the amp

Here is alink to the manual:http://www.behringer.com/EN/downloads/pdf/DJX750_P0956_M_EN.pdf You should have no problems hooking them up (RCA to RCA). I've been using a Behringer XenyX1222FX board in my recording system for about 4 years now and have been quite satisfied with it's reliability. Try to find a dealer who will demo the mixer for you.

You will need at least a 24 channel mixer to give you spare channels. You also need an active crossover and a snake. The advice you need is too much to include here so I am including some links:http://mixonline.com/live/FixIts/, http://en.wikipedia.org/wiki/Live_sound_mixing, http://mixinglivesound.com/blog/tag/outdoor-concerts/,http://mixinglivesound.com/blog/, and http://www.scribd.com/doc/11435651/Beginners-Guide-to-Mixing-Sound. You may need more monitors than you are planning on and you might consider in ear monitoring.

Come from the main outputs, probably labeled L/mono and R and connect those to your mixer for your audio system on either stereo mix input OR two two inputs that are panned opposite.. DO NOT connect this to a consumer type stereo system as you may blow the speakers or amp. Consumer stuff just isn't able to take output from music equipment such as this UNLESS the volume is kept very low.

just connect a baby plug to rca from the pc to the mixer respectively

This is a wild educated guess.

I repaired a Behringer guitar amp that had a DSP effects in it (BX1200 I believe).

The effects were on a seperate board and the board had its own 5 volt power supply. The filter capacitor for the 5 volt power on the board was UNDERSIZED and it had degraded causing the 5 volts to drop out of regulation, dipping to 4 volts. This caused the DSP effects to fail.

YOUR device MAY use the same DSP OR design.

Check the 5 volt power with an OSCILLOSCOPE to look for dips below the 5 volt operating point that occur at 120 HZ.

Send main 1 out to your Crown. Jump Mon send to PWR am insert.