P.S. Yes, the guys did do longwave "DXpeditions"
to quiet locations. 
I heard many Longwave broadcasters during good conditions.

Since we didn't have SDR and only rudimentary digital 
modes, most filtering happened "between the headphones."
Most of us built analog 40 cycle bandwidth audio filters
centered on 750 cycle CW tone.  "Ringing" in the filter was
a problem if one over-drove it.
As with any such filter use, knowing that your receiver
were and the subject beacon were
exactly on-frequency was crucial.  We didn't have
GPS-disciplined anything, so we used known on-the-air
signals.  My favorite was the 3rd harmonic of WWVB,
which gave me a precise 180 KC carrier and a 750 Cycle tone.
  Longwave broadcasters
and known-correct NDBs like DIW in NC and TUK
could also be used.

73 Dave AB5S