The low impedance of your can is the problem. The efficiency is not relevant to the ME's output impedance potential problem (the efficiency is more related to the ME's power output, but it's not an issue here).
If the Samson SR850 have a (relatively) flat impedance response across the audible range, then the high output impedance won't change the "intended" (original) frequency response of the cans.
If their impedance response varies a lot with frequency (as occurs on Balanced-Armature IEMs, which have cross-over network with complex impedance, that by nature, varies with frequency), then the intended frequency response will be modified (in general, for the worst, but not always).
The amp's output impedance also determine electrical damping. Simply put, the higher the damping factor, the quicker the driver "reacts" (= the more the amp has control over the driver). If the damping factor is too low, the driver will be "loose" and bloom will occur (e.g. in the bass).
The damping factor is defined as DF = Zcan / Zamp. As rule of thumb, DF should be >8. Which means, if the amp has an output impedance of 50R, the can should have an impedance of (at least) 400R.
Wow, thank you for the detailed explanation. I actually expected something something a little less....expansive? Oh well, it's a good learning experience. Anyway, thank you for telling me that, now I know not to get low impedance cans for that device.