I would use the effect return. So you only use the Poweramp of the Guitar Amp if you use the regular input you also get some of the preamp color if you know what i mean.
For the regular signal to FOH you should go directly into the mixing desk. But i think this one was clear 
If you're talking about 'The Clash of the cover bands', I had the same issue 2 years ago. I was not able to go this route with the provided amps. They were not able to manage looping in the KPA into the effect loop of the amp and get any sound out of it. In the end I had to use the amp as it was provided which was a nightmare sound wise.
HTH
If you plug to a return, don't forget to disable the cabinet.
You might want to prepare a performance with amp and cab disabled in case you have to use the front input of an amp.
Worth checking the amp provided has an effects loop as it's not a given.
