I agree with everything voxman55 stated.
In my case there are three situations (venues) that I deal with: Home studio monitor, FRFR amp in church orchestra, regular amplifier in rock band. For the first two I use the MAIN Out and AMP OUT for the last. Under Global Settings, Main Out Level = Line and Amp Out Source = Pre Cab/IR. Within the preset the Cab is last (to the right) in the block chain. All my effects, exps, amp blocks are to the left of the cab. (Note: Make sure your Cab is active in your preset, if you want it on when playing through an FRFR. I made this mistake thinking something was wrong with the Pod Go.)
When using your external amp, do a test by enabling/disabling the Cab block in the Pod Go. There should be no difference in sound and will insure the Global Amp Out Source is set correctly.
This is not related, but another thing I do is run my AMP OUT through the amp Return and bypass the external amp preamp. I also use the Pod Go Preamp blocks instead of the Amp blocks. Not sure if it makes any difference but it seems like replacing a preamp with a preamp is the proper thing to do. Just my opinion.
One thing I'm working on is trying to duplicate the sounds between these venues. For the FRFR/Monitor situation, I tried to select a Cab that best fit my external amp cabinet and tweaked around with the mics. Also fooled around with the Preamp settings. Still not there yet. If you have any suggestions on the best way to do this, I would love to hear it.
Good Luck!