I've recently received my new iMac 2017 27" and, like EugW, I have another iMac 2010 27" to compare. The sound between the two is clearly different, but I would not say that is worst in the new model. The bass response of this new iMac 2017 is a lot better than the oldest that I have. I was comparing both side by side and, although the highs frequencies are more present in the 2010, the bass response is a lot better in the 2017. Perhaps this is the sensation that all of you are perceiving.
I'm used to work with high quality headphones and monitors (I work as composer and produce orchestral mockups) like AKG, JBL, etc. Some of them costs hundreds of dollars, and they are only headphones! But I remember the first time I bought a studio headphones (AKG K240 MKII) and I was very concerned by its sound... The first days it sound to me very bad. Until that moment, I was only used to listen "hi-fi" systems, where specially high frequencies are altered to sound more "impressive" or "present" to general customers, but this "hi-fi" sound is really not natural. When you enter to the music production world, the first thing that you try to get for work in good conditions is speakers and headphones that sound as flat as possible, and always avoid that "hi-fi" sound with high or bass frequencies altered in consumer speakers. Well, after many years working with "near to flat" equipment, now my ears prefer all these kind of speakers and headphones and its very difficult to me to listen with bad quality equipment. When I try to listen again with my old "hi-fi" systems or headphones, I really feel uncomfortable and thinking "how could I like to listen music with this before?"
I guess people get used to listen what they have at home and when something new sounds different, they usually do not like it. I'm not sure if those who are saying here that their iMacs 2017 sound bad have really a problem in their units, perhaps there are some of them defective. But I think its also possible that this different sounding that I have noticed between the new model and the other I have from 2010, with more bass that also camouflages and balances the highest frequencies, is really the little difference that cause your concern. But I repeat, for me my unit is not bad, just different, but good enough. And of course if I need or want more, I change to my studio monitors or headphones that any iMac can't beat.