RR NDB
Regarding the long wave stuff - that's REALLY simple minded stuff. All you are doing is reducing the bandwidth on an amplitude detector. A factor of 100 reduction in bandwidth (say 30 Hz from 3kHz) gives you that 20 dB you talk about.
Unfortunately that does not apply here. The pinger is like an old time radar pulse, short and sweet. It has no spectrum spreading that can aid discrimination with a low power pulse spread over time. It has no serious possibility of long term averaging. So that equipment you write about is just about useless.
And that is probably what Thales tried to do with the submarine records. There's no magic they could pull out of recordings. The pinger pulse is too short.
{^_^}