There is no fixed “single” XBass, it is dependent on the model. It is not “Basshead” type bass boost that boosts bass in an unnatural way, instead, it is meant to compensate low bass deficiencies in headphones.
It is best to try with some music that has very low bass.
Example: The main theme from 2046 Soundtrack (especially the “Train Version”) or anything with very low organ notes. XBass will not (for example) make a kick/bass drum louder, but it will give it more “kick”
Please also view the spectrum of the iDSD micro XBass with and without.
+4dB at 30Hz, around 6dB at 20Hz. This is a substantial boost, but only for very low frequencies (below the kick drum of most rock/pop drumkits.
Further reading with regards to XBass: https://ifi-audio.com/wp-content/uploads/data/XBass_TotallyAddictedToBass.pdf
For some self-assessments you can try to download RMAA (http://audio.rightmark.org/products/rmaa.shtml) and to do a test into any kind of analogue input of a sound-card on a PC/laptop (line-in, iDSD micro in either headphone out or line out with pre-amplifier) and to run the frequency response test.
