Backfires are not really a sign of bad fuel, it's unburned fuel igniting outside of the combustion chamber. If you are tuned at all you are most likely running very slightly rich as it is safer for the car. Our cars run rich slightly between shifts as well which cause backfires. If you run a true bov then you are even more likely to backfire.
Bad fuel causes misfires and knocking/pinging. Kind of weird that it is only happening at certain RPMs for Celery.