It's not the HS5 really, it's the HS8s subwoofer. When I'm saying that there's no bottom, it's of course with reference audio material, a commercial production in more or less the the same genre. Say, Rammstein. The listening hardware and the room environment are important, no-one will doubt about this but, they vary so greatly that it's up to the music to adapt to the listeners and not really to the listeners in general to adapt to the music.
We, who are doing stuff with audio can tailor the listening experience. So will audio enthusiasts. But that's not the 'market' at large though.
According to mixing/mastering engineer Michael White (Whitney Houston, David Bowie, David Byrne, James Taylor, etc... i.e. not according to me) there should be a presence of each instrument between 400Hz and 2KHz. For basses that would be a transient. Same with kick drum. The music should be heard reasonably welll (meanign able to distinguished instruments) in a car with windows down at 80km/h as well as basses should be present for equipment that have the means to deliver basses. And that can include a kitchen CD player with a bass boost button. All in the same mix.
The other aspect underlined by Mike is that exactly like objects that we do not fully see, eg. the hood of a car showing past a building without being able to see the rest of the car, the brain will reconstruct basses from the tip of a bass sound. About this though I haven't yet figured out the exact technical mixing means to get to it yet. And then there are mixes that are made to get better as you crank up the volume. The idea is to get the listener attention on radio and when he thinks that's a good tune and raises the volume, the mix will open up.
I do not have earbuds since I do not have a device for that, but I have a single Behritone (mono) to which I switch to from Mixbus32C (Ardour) to hear how it sounds in total midrange on a single speaker designed to be relatively crappy. I run mixes through that. I also give a shot using the kitchen CD player. In addition to the regular Yamaha monitoring and Sony headphones. At least I try to do it most of the time. And then also a run in the car CD player. With windows down when it's not winter. Not on all mixes, but when I have the time, the idea is to adjust the mix from average perceptions.
So I don't think that having no idea about flat responses and rooms matters really, in general. There are countless of commercial productions in every genre to compare with as reference for frequency responses. This said, if the artist wants really to make it sound thin for artistic purposes, then that's also all right. But it has to be a conscious choice, otherwise the mix/master might be lacking what listeners of that genre are expecting, in general.
I got the new version, I'll give it a try a bit later.