Peavey XR series 600 may have been left on without one speaker plugged in, what is the likely diagnosis?

That is standard UK wiring.

Brown in Live.
Blue is Neutral / Return.
Green with Yellow stripe is Earth / Ground.

https://www.google.com/search?q=peavey+xr+1600d+wiring+diagram

Manual

https://peavey.com/manuals/80301359.pdf

..

If it has a effects loop output and input on the p.a. head and a in and out on the reverb effects processor, then yes you can, just run the effects loop output signal into the input on your reverb processor, out of the output back into the input effects loop on the p.a. head, kinda like you would a effects pedal thru the effects loop of a guitar amplifier. hope this helps!

You can get them from Peavey. Heres the link-
Peavey com

does the speaker still work? use a 9 volt battery and do a test thump to see if the horn and woofer still work...when the power amps on these units fail most of the time they shut down.

the pa1 or the pa2 input jack on the front of the unit is dirty, or you have a cracked solder somewhere, as these units are gettting old.

When you unload one amplifier you stand the chance of killing the amp's output transistor. If you must use only one speaker, you have to bridge the amps.

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?

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.

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.

Peavey XR 8600?? Well it's 600w per ch. at 4 ohms/per side. So, whichever mode you use (Main, or Main/Monitor) you are best to have a 4 ohm total load for each channel of amplification. Ex.--Mains only, one 4ohm speaker per channel will give you 600W per side. Or two 8ohm speakers per channel in parallel, which makes a 4 ohm total load.

Main/Monitor mode---treat it the same way. one 4ohm, or two 8 ohms parallel. per channel.

Your best situation is to have a 4 ohm total load for each channel of amplification to get your 600W.

Parallel-- 8ohms each / 2 speakers = 4 ohm