It is possible to render the envelopes to audio and see exactly what they do and where to fail .
You create a dc sample( positive energy only ) and make sure that in the song settings the track headroom is set to 0 db .
Basic amplitude modulation really .
In all cases renoise ads a 2ms small section to the attack stage ( linear interpolation ) ,
See xrns and render to selection ( there are different presets to the envelope ) .
Fast attack ( 20< ms ) to full range (0 db ) is problematic .