I've been using XBA40i for more than 2 years. I am happy to let you know that Z5 is definitely worth the upgrade. I mostly listen to pop, jazz, classical (organ, chamber, opera) , blues on ALAC, FLAC, WAVE, DSDIFF files with no equalizer enhancement on my Sony NWZ-ZX1/PHA-2. Here is my initial impression after 40 hours listening with the MUC-M12SM1 unbalanced cable ( highly recommended for A3, A2, H3, H2 owners ) It gives better control of the low frequency (tighter and deeper bass) and more presence to your listening experience than the supplied cable.
Comfort - A sizeable in-earphone but lightweight, very comfortable for long hours listening
Soundstage - Airy, clear, good separation between instruments and voices
Treble - Crystallized treble with plenty of details coming through without being clinical. Very good clarity. No sibilance. Plenty of headroom without being fatigued
Mid - slightly warm mid, clear, engaging with plenty of presence factor, no mid bass bloat to voices and instruments
Bass - Tight, well controlled , clean, impactful when needed, depth of bass
Z5 requires a running in period in order to work to its best ability. It keeps improving in all areas! It really shines when listening to high res files! ( Best with Sony PHA-3/ MUC-M20BL1 balanced cable )