I overlaid GoldenEars' FR of the MK5 with the same ones they did for the ER4PT with P/S adapter attached.

For the depth that GE inserted the MK5, it doesn't quite have the treble bandwidth of the ER4S, but it has quite desirable bass qualities.

The MK5 and EK5 do not measure that similarly; while they show similar ringing at around 5 kHz, CSD plots show that the MK5 rings less, <1 ms compared to >1.5 ms. The MK5 also shows greater evidence of natural group delay in the bass (though still a little overdamped).

The bandwidth falloff of the EK5 suggests that the high impedance voice coil is prohibitively inductive at higher frequencies, whilst the MK5's 32 ohm voice coil is able to get to 16 kHz (sorta, not quite), at right about the testing limit of these IEC 711 couplers.

My instinct is that the MK5 is in fact a better buy over the only slightly cheaper EK5. It remains to be seen whether a capable amplifier will be able to overcome the inductive voice coil of the EK5, but doing so would essentially defeat the purpose of the EK5, which is to attenuate SPL to safe levels for children.