What about EQ-ing LS down, while keeping the rest at 0 and then proceeding from there?
EQ can alter the freq balance a bit. And on freq plot - while I think it might be useful to get the measured FR, I still think it's better to go with the perceived response for EQ-ing. As you know, there are various factors like ear canal shape, depth of insertion, tip, Freq response of the IEM and most importantly, FR of our own ears which determine how an IEM sounds to us. May be you can run a basic frequency sweep to determine how it sounds to you and then proceed from there.