http://rinchoi.blogspot.com/2012/10/heir-audio-4ai.html
Read the con section. I know this is for a different IEM in the past, so who knows if they have learned since (but I wouldn't count on it.) When you have to link multiple BAs for an IEM with each BA responsible for different frequency range, you need to design a network to correctly link the BAs together, and hence the "crossover network" when sound crossing from one freq range to the next. In fact, Dr. John Moulton started Heir then moved onto creating Noble (you can look the background up
here if you want.) And let just say that I trust Noble much more than the other company.