The Sonobuoys that will be dropped as listening devices in the larger search area are indeed broad spectrum passive listening devices. The modifications to the aircraft equipment might allow for a narrow frequency spectrum to be more closely monitored so that the returned signal will only scan the chosen frequency range around 37.5kHz
The modifications to the aircraft were to temp-fit new sonobuoy receivers and software capable at looking at the extended frequency range of the sonobuoys they are using.
The sonobuoys continuously transmit their wideband acoustic data which is received by the aircraft. The crew will then focus their attention in the higher frequency bands but all frequencies are available for display at the receiver.