I received a pair of HiFiMAN RE400i earbuds ($79) recently, to use with my iPad when I watch movies.
The first thing I noticed was that the sound was very clear and detailed, with no harshness, even with the volume up reasonably high.
However, the bass was lacking.
A bag of various sized rubber tips came with the earplugs, and I wondered if removing the default tips, which were small, and replacing them with the larger ones in the bag might solve the problem.
In the photo below, you can see the earbuds with the larger rubber tips mounted. The default, smaller tips, are sitting next to the earbuds.
Well, this is one of the quickest fixes I have experienced in a long time. The bass was much louder, matching the volume of the midrange and treble. Now I can experience the full extent of explosions in war movies, and also the bass section of the Boston Symphony Orchestra.
The explanation is simple. Bass reproduction is best when the enclosure is large. So, using the larger eartips increased the “enclosure” size (space between the driver and your eardrum).