After reading related chapters from Uric's Princleps of underwater sound, Rip's statement looks solid. It's simple matter of what is dominant in the signature.
Without cavitation NB signals dominate total radiated spectrum, even for slow moving surface ships. NB will have major advantages, by telling what is noise and what is signal.
With cavitation (which is normal for surface ships at all but minimum speeds) the broadband noise dominates and covers all the lines. In such case NB will be of little advantage, if any.
Still exact amounts are mystery .. and big no-no from experts I asked.
