After reading this again, this line feels not right here
The EngineChannel is instantiate an then added to MixingEngine: 
pMixingEngine->addChannel(m_pChannel);
https://github.com/rryan/mixxx/blob/features_effects/src/basetrackplayer.cpp#L40

We should keep this way here as well:
pass EngineEffectsManager and register EngineChannel from outside.