Finally found a great kick drum sound using a Doepfer A-105 and pinging it with an ADSR.
The problem is the resulting kick drum sound has an unstable (intermittent) positive voltage in the artefacts of the resulting sound, making it useless to generate envelopes to duck VCAs.
Has anyone got any tricks to make a more 'stable' sound. Quick term fix seems to be a low pass filter (set very low) before the envelope generator, but I'd rather be able to fix the kick sound itself.