Be careful about comparing the seawolf to the 688, they are no doubt talking about the unimproved 688, not the 688(i). Checking the LWAMI DB (and assuming I am remembering how to do this

)
....................Seawolf.........688........... 688(i)
SL@ rest...........55..............62.............58
SL@ 5knts.........57..............65.............60
SL@ 20knts.......64..............73.............68
So under LWAMI the Seawolf at 20 knots is already very close to the sound level of a 688 at 0 knots. (64 v 62) All those numbers (I believe) presume you are not cavitating.