I hardly ever if never use the flash for stage shots...
Spoils the colours....
I use mostly 800 to 1600 iso with my S3,
not really tried my S5 yet at 3200...
But band lighting isn't like theatre lighting
and won't be as good...
I'd suggest a weak flash to try and pick
up some of the detail. Depending of the venue
of course, you may be able to bounce the flash
off the ceiling..





).