I can confirm Mike's (G3XDV) conclusion that even a simple attenuator can
improve the receiving condidtions a lot.
Some months ago I took advantage of a slow-CW transmission of G3XDV (with
his old VFO that had the very weak 'FSK-trace') to 'optimize' the
attenuator value.
Using my inverted-L as receiving antenna, with 25dB attenuation the
reception of the weak FSK trace was optimal (100% detectable) while with
0dB attenuation the trace could not be seen at all.
The RX used was (and is) a Kenwood TS440 direct at 136kHz and with a 500Hz
filter. Using the notch and IF-shift I narrowed the bandwidth a bit,
allowing sufficient attenuation of DCF39 while receiving slow-CW at 137.7kHz.

73, Rik  ON7YD