There is possibility of greater treble bandwidth. Doesn't mean all multi-BAs have better treble bandwidth but they potentially can. B2s for example have greater treble bandwidth than every tested single BA i've seen. But certain multi-BA IEMs like the Shure SE535/425, XBA-2, SM2, etc. Have even less treble bandwidth than a single BA ER4S or PFEs.
Though in the end, the treble bandwidth superiority will be very small. ER4S goes up to 17k, PFEs to 19k the B2's superior treble bandwidth goes higher than 20k, but 20k is already pushing the hearing limits on many.
Better response or "resolution", nope no advantage. I think placebo and bias play a role here.
Edited by Inks - 8/7/12 at 11:23am