Some of my favorite IEMs are single DD models, the Final A4000, Final E5000, JVC HA-FX1100. The pair I use the most is the Sony XBA-N3 which is a hybrid, it has a single DD and a balanced armature. I have some other hybrids that have a lot more drivers that I rarely use. I'm currently very interested in all balanced armature IEMs after trying one recently.