I ran some tests using Bob's signal from last night using
various sferic blanking thresholds

 http://abelian.org/vlf/tmp/29499_140303a.png

I've given the threshold in dB as you would set into Spectrum
Lab.  This is the ratio of the trigger threshold to the mean
signal level (maybe SL uses RMS, but not much difference).

The percentage indicates the proportion of time that the
blanker is active.

The high threshold of 30dB is enough to bring the carrier
above noise.   Morse was being transmitted and with 10dB
threshold the sidebands are starting to show quite clearly.

I don't think any more signal features appear with settings
lower than 10dB but maybe the noise floor drops a little.

The setting I've been using at 29499 is 3.5dB.   At lower VLF
I use 1.6dB.   It looks like we can be less aggressive at
these higher frequencies.

The input bandwidth to the blanker is 28 - 31kHz and there
are no continuous signals there at my location.

One related point - ensure that nothing in the signal chain
(analogue or digital) is clipping the signal, except maybe
the occasional sferic.  In particular if the mains hum
hits a clipping threshold anywhere along the way, it will
intermod with the noise and prevent any decent reception.
Instead of a nice smooth background hiss, it will sound like
a rasping buzz at all frequencies.

--
Paul Nicholson
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