XR 8600 what color wire on the output connects

You would come out of the Main output on the front of the mixer into the line in on the Bose.

You may find this site useful- L1 Unofficial Performers Guide

Is your graphic eq assigned to main and is kit set correctly?

Yes.
The only thing the power does is power the speaker outputs. (Main out) or (aux out if available)
so if u want to just use the headphone jack or whatever it is your doing your not going to hurt the mixer by turning it on without speakers connected to it.
Q: What is it that u are doing?

DO NOT parallel more than two 8 ohm speakers on either output and you will be OK. You could also use one speaker on each output. That would be easier on the equipment as it would be an 8 ohm load on each instead of a 4 ohm load on one of the internal amps. If you want to drive stage minitors, then you existing setup is fine and you will connect the monitors to the second speaker output and set the Mon switch as MAIN/MON.

If you run the speakers FROM the main you should have BOTH the effects and the EQ invilved with your signal.

Iis ALL your equipment powered from the same receptacle? It had better be... ground bounce can destroy input circuits in these units. I don't have the schematic but a common problem where XLR connectors go into a mixer is they have radio-frequency noise suppression caps to ground from each side of the XLR input. These caps are often lower voltage rating and can be popped by ground bounce or ungrounded equipment being connected before the ground connection is made. When one goes, there is loss of half the voltage or 6 Db of power gain. Since these RCA's are right at the XLR they MAY connect and have these caps and one may have shorted. That means the repair shop I am afraid unless you are adept at electronics. Most of these mixers are a pain to work on that area as all the knobs and screws have to come out/off to get the circuit board away from the panel to work on... I wish I had the service manual to give you more info...

PLEASE in the future power all equipment from the same receptacle or power source AND for units that don't have three wire power cord, NEVER change cables that can break the ground connection such as the RCA cables while power is on to any of the equipment. This will save repair bills...

This is from your friendly electrical engineer that works with this stuff and repairs his own...

That's easily done. Simply connect the left(or right) output of your mixer to one of the channels on the peavey. Use any line input. Since the peavey is a mono system, there's no need sending a stereo mix to it.

First off it's outputs to your speakers. If you had an extra set of powered or amplified speakers you could "Y" cable your speaker outputs. Your passive speakers won't work without an amp. You really need powered speakers or an amp for your extra speakers. (if you had powered/amp'd speakers you could run them off one of you "aux" out for monitors. Do not daisy chain your speakers connecting twi together, you'll change the resistance/ohms and damage your speakers. Trust me, I do this for a living.

Sorry, but this unit does NOT support "bridged" mode. You will have to drive seperate speakers from the two seperate outputs. You would NOT get any more power if you could do bridged mode than you could with 4 ohm speakers on each of the two output channels.

The monitors will have to be POWERED monitors with built in amps.

You can simply use a wye splitter cable to drive the two monitors. Run the cable from the mixer to one monitor and put a wye cable and an extention to the second powered monitor speaker.