To some degree that is not true. Yes, while you can have bass and details you can't have every frequency sound balanced if any one frequency is above the rest. This isn't an opinion either, this is a scientific fact with plenty of audio science backing it up. When most people have a bassy headphone that also has details it's because the treble is boosted as well. This is usually a V-shaped headphone. You lose mid details here. So while things might sound "crisp" and "detailed" you are actually losing "mid detail". Some people like this. There's nothing wrong with that. But the fact is that the bass and treble are "masking" the mid frequencies.
You can have something like the mh1, which has pretty good smooth flat frequency response, but extra bass. This sounds o.k., because the whole spectrum is smooth, but there is still slight masking of everything above the bass. It may not be extremely noticeable, but as you raise a frequency area further there will be a point where it is more noticeable.
That isn't the whole picture though. There are audio properties like decay, delay, impulse response, distortion, etc. These all affect the sound, sometimes in ways people wouldn't expect. For instance, having more decay in the bass area can give the bass a fuller or thicker sound, which some might think sounds "bassier". However to get an real audible difference you need the increase of the bass frequencies, so there is a difference there. But even if it did sound bassier to someone, by the time you added enough decay to make it truly sound noticeably bassier, say by 10db or something, it would sound like crap from the amount of decay. But that's not even really possible, it's just an exaggeration to show the point.
So if you want extra bass you lose a bit of everything else from the bass masking everything else. To get "true" detail of the full audio spectrum all of the frequencies need to be equally portrayed. Now, you can have some degree of bass or any other area boosted or cut to a degree without really changing the overall sound "that" much. This is why there is a debate over earphones having +6db of bass to replicate the sound of a speaker. The pro is that you get more bass, and to some people that "feels" or "seems" more like the bass impact of a speaker. To others, even if this is true, they may find the bass masks the other frequencies too much.
Anyhow, don't mistake "an iem having a lot of bass" with "an iem being able to reproduce a lot of bass". These are two very different things. When I say bassy iem, I'm talking about an iem that permanently has all of the bass frequencies boosted. Everything that you hear through them will have more bass than neutral. On the other hand, if a very flat iem has good "extension" down very low, and a song demands a lot of bass be reproduced, this earphone will pound out that bass, but when no bass is present it will not boost anything else.
A lot of iems lack truly good sub bass performance, because they don't extend that low. Some iems accept that to remain flat. Others boost the whole low bass region to compensate. If you left it boosted, things would sound muffled and warm, so they usually boost the treble to "bring back the treble". This is really bringing the treble back to or closer to the bass level. This leaves you with lower mids and thus a "V" shaped sound.
So, anyhow, as I started saying, in general there are some things that can change the perception of the bass a bit, but overall frequencies are either boosted and mask other frequencies or are reduced and allow other frequencies to dominate or are balanced and allow all frequencies to be heard equally. The last thing I'll mention is our hearing.
Humans are less sensitive to really low sub bass. So boosting the sub bass a lot may not result in our "hearing a lot" more sub bass compared to boosting other frequencies. These are also the frequencies that really give bass that deep kicking power. So while they too will mask things when they reach a point, they can be boosted to some degree with less obvious masking than other frequencies. And since most iems lack real low sub bass anyway, boosting this area is actually bringing it closer to flat on some iems, such as the er4s for example as that lacks sub bass compared to the rest of the spectrum. Whereas something like the xba has good extension and sub bass already, so boosting it would start to mask other things.
