My experience is focused on web usage, but last time I checked OpenFL did not support sample callback to push data to audio buffer. Not sure how Kha handles sound, but the sound API seemed quite basic.
Heaps would probably be your best bet, and it seems like it should be easy to add your own DSP.
For web it is more difficult. Heaps can use script nodes in WebAudio to emulate OpenAL, so in theory it should be great, but script nodes will cause memory leaks in many browsers. So unless you can stick to existing WebAudio processing nodes, it is very problematic to do serious audio applications, and will probably remain so until AudioWorkers are available.