In the brief time that I tried them, I agree that the 940 is better. However I won't say it is "much improved", in audio one typically has to pay many times more to get anything "much improved". The 940 is more detailed and refined to me. Take the 940, add more mid bass and take away some details and airiness, you get the 840. Although their basic sound signature are similar. I don't have problem with its bass, my only complain is the treble can be a little sharp sometimes, the 840 is the same in this regard.
I also disagree that the 940 is low fi, I'd put it just under the DT880 and the like, quite easily better than DT770, and to compare a closed headphone to an open one is not quite fair.