Many WWII bases didn't have bunkers... They had ordnance dumps, arrayed around a base at a safe distance. In some cases, the dumps were more than a mile from the airfield. Different sized bombs in different dumps. They were usually heavily netted and camouflaged. Where bunkers were constructed, they were usually made from cast reinforced concrete, and then covered with earth to hide them from view. When the 9th AF fighters moved to France in July of 1944, they would store ordnance in barns, in the woods or anywhere else easily hidden from aerial view.
Having the dumps hidden obviously won't work in the game. So, if everyone knows where the ordnance is, it seems that the bunkers should be hardened to make it difficult to blow up as an offset to being hidden.
Another thing to keep in mind. Hardening the bunkers may have a significant affect on game play. Today, any three day noob can take a 190A-8 and kill the ordnance at three airfields with guns alone. The impact on game play is substantial. If each bunker required 2,000 lb of bombs to kill, a single fighter would be hard pressed to be able to kill more than one per reload. Bump it up to VH hardness and you'll need bombs and rockets to kill one bunker, assuming perfect aim. What this does, is eliminate the griefers porking ordnance, simply because they want to have a negative impact on the game.
One more thing... When dumps do explode, it might be interesting to define a blast radius of, say, 200 and 600 yards. Anything flying within 200 yards is blown up. Anything within 600 yards takes random damage (regardless if being friend or foe). With that in mind, shouldn't rearm pads be at risk? Obviously, there's ordnance there.