Hello! This study seems very interesting. I am not familiar with visual and audial modalities as an object, but know something about working with breath as an object. There are some meditation frameworks that strongly connected to the breath-say 9 stages Shamatha practise. On later stages some special breath-related technics used there in order to achieve the higher «resolution» and attention «volume». (I mean breath following and body breathing technics). I can’t imaging how it’s possible (and is it possible) to use it with different modalities within this framework. So, in my opinion, though choosing the «right» modality could be helpful in earlier stages to help to stabilize attention, it could the obstacle in later stages (at least in some types of meditation). What do you think?