If I am not mistaken, it's a fact that a single driver is enough for over the ear headphones to get a full range, transparent sound because they only need to work in the small acoustic space around a person's ears. In case of IEMs, it seems even more obvious to me that one driver should be enough, as they only have the tiny ear canal to work in. I do think that single driver IEMs haven't reached the sound quality level of the best full sized cans yet, but they should in the possibly very near future, with planar IEMs rumored to be in development right now and with further development of electrostatic IEMs. Who knows, maybe Beyer may even develop great sounding Tesla drivers for their IEMs, or Senn might develop a successful miniature Ring Radiator driver... As for multi driver BAs, they may also have a future with all those fancy phase alignment technologies coming out, but ultimately I feel that single drivers will eventually replace multi driver BAs in IEMs, at least due to the former ending up being less costly to implement. As for current BAs without phase alignment technologies, I feel that they suffer from audible phase problems that, from my experience, is the greater offender than the, fairly minor IMO, lack of extension, resolution or transparency of current top single driver IEMs.
Edited by Pianist - 5/4/14 at 12:33pm