So to summarize, it doesn't matter whether the right or left transducer is in your left ear, you (and you alone) get feedback. If so I can't imagine its anything other than a fit issue combined with how the unit senses external noise.

Do the headphones allow you to switch earpieces (e.g. my shure earbuds allow the use of 3 different sizes of black cones, two sizes of foam sleeves and silicon cones)?