The most important factor in making sure that your earbuds fit correctly and that the sound (especially bass) is optimal, is choosing the correct size ear tips for your ears.
The M100 comes with small, medium, and large silicone and foam ear tips included. We would recommend trying the full range of tips provided, so you can find the right fit for you.
When trying the foam tips, we would suggest gently squashing them up with your fingers before inserting them in your ears, as this will allow them to naturally expand to fit your ears. Please do be careful that you do not push them too far in your ears when inserting them.