The organ mix already has a positive weight applied, this needs to be increased a little.
Ended up reducing the synth and string mix. The issue is that the synth already thumps out a fair signal and I don't really want it to start clipping.
Fix for 0.40: 2805405, 2805407
Attached the engine file to resolve this issue and 2805407 for any 0.40 release.
This was integrated into 0.40.5.
Resolution will be in 0.40.5, the relevant file is attached to this bug report.