Chris in texas. NRR 33 are about the highest rated plugs available. According to the Australian OSHA Plugs are more effective than muffs. I trust their data more than US data. A combination of plugs and muffs give increased protection but the increase is not linear.
Single use foam plugs are the most effective if they are properly installed. As you move they maintain full contact.
Muffs, on the other hand, do not maintain full contact and their protection is degraded. For example: If you wear shooting glasses and the bows go under the muffs then you loose protection. Some studies indicate the lose is as much as 25%. In effect if the Muff's advertised NRR is 33 the effective NRR is 24.75.
There is a graduate level paper in this discussion. I hope this is helpful. If you would like a more complete discussion PM me.