The stereo image from mid side recording can be very sensitive to exactly where the mic was placed relative to the source and how accurately the mic was directed. Maybe the imbalance you are hearing is due to that. To check the device itself, you could for instance set it up carefully in front of your hifi, correctly centred and aligned, then record a mono playback, centred first, then slowly panned across with the hifi balance control. Check the replay on headphones.
It's worth mentioning in passing that MSED can also be used at one pass to convert any signal from L/R to M/S and back, manipulating width and balance as required. I guess the key difference between real M/S and M/S created from L/R is the polar patterns of the mics, when the centre of the mid mic would capture a fully on-mic sound from the centre of the soundfield. A crossed pair captures the centre of the soundfield slightly off mic of course.