The current way auras interact means that combining a weak aura with a strong aura can actually result in a sum that's less than the strong aura; worse, the interaction between different ranks of aura of radiant protection seems to make block not function. This appears to be a result of a balance pass some years ago to prevent auras from being straight up additive, which was kind of overpowered even back then and with teamups could actually result in exponential attribute gains. Unfortunately, it appears that the method used doesn't work.
My best guess is that it's one of things: either the auras have a check for the total number of copies of that aura and reduce their own magnitude, or when an aura is applied it checks whether the target is already affected by an aura, and if so reduces its own magnitude (this would make the sum of auras order-dependent). Doing either of those things is somewhat complex, so it's not too surprising that it gets things wrong.
A simpler and more reliable method would involve two buffs, one of which could be invisible. One buff would be owned by the aura source and does nothing but track magnitude. The other is owned by the aura target (therefore unique even if multiple aura sources; this should be a separate status effect, not something owned by the individual powers), and every time it refreshes it calculates the proper bonus based on the magnitude buffs above. There are several ways to get sharp diminishing returns, such as the nth root of the sum of the nth power of the individual buffs, or (more radical) auras could be revamped to have diminishing returns in general, and 2 300 Pre auras really do add up to the same as one 600 Pre aura.