If it's worth another $100,- you can scan for frequencies in use with this device:
http://micro.arocholl.com - You could maybe even rent them.
But I doubt if you'll get a decent recording, monitor mixes are filtered and don't sound like the audience mix.
Besides, different brands use different squelches, and G'NR probably doesn't use the budget EW system.
Another thing, radiomics are geo-blocked, so you'll have to use a receiver from the same country the band or it's PA company uses.
I'd take a mic..