I would think that you are overpowering the ability of that cab, given the new drivers. If I recall correctly, (I have that cab in the basement!) there is not very much internal bracing in that cab. Those JBLs are probably stressing the cabs based on weight and performance.
There is a thread on here somewhere where on of the members here filled one of the speaker holes and ran the cab with just the one 15 speaker. He claimed good results.
Otherwise, I would defer to folks who know about speaker and cab parameters who could help you out.
Yamaha Bass club member 157 - BB1100s, BB605, Aria owner, Yorkville/Traynor member 62, Roscoe LG3000, Pedulla Rapture