I had the chance to compare the XBA-Z5 and the XBA-A3 on 3 separate occasions, with the longest demo session being about 30 minutes long. After the first demo I started reading up on various opinions of the Z5, so please be aware my impressions have been influenced by this discussion and talking with Tom Tsai of CYMBACAVUM, who shared his own impressions of the differences between the two.
Here's my take.
They are very similar sounding earphones, with the same basic character. A big bombastic bass line coupled with an extended, well layered and detailed treble section. Between those extremes: slightly subdued mids, particularly around male vocals. Package it all up in the expansive soundstage that seems to be characteristic of the Sony ear hanger designs.
The XBA-A3 has slightly more forward treble with more of a shimmer to it, and this makes the A3 sound a bit more open and a bit clearer in the vocals. At times the treble on the A3 can have a splashy or metallic feel to it. The Z5 is more diffuse and distant in the high frequencies, but you might feel it is better integrated depending on your tastes.
The XBA-Z5 has slightly more subdued vocals and sounds darker. The bass also sounds tighter on the Z5 - same quantity, just a slightly reduced feeling of bloat. It has more of a visceral impact, more of a deliciously tactile feel. In other words: dat bass... *drool*
Both earphones sound good to me and both sound better than the XBA-H3, though I think the A3 is closer in character to the H3 - particularly with the treble.
I don't actually think the Z5 sounds 'better' than the A3 other than having more solid and controlled low frequencies. It certainly doesn't sound anything close to better like the price difference would suggest. Depending on how you like your treble, and how much you like your bass, you may prefer the Z5 or A3, or neither.
I ended up buying the Z5, but not really based on its performance. I had $6000 HKD of tax rebate money and just decided to spend it on the Z5 because it has a beautiful build that I would be very pleased to own as a collector. If the A3 sound was in the Z5 shell and vice versa, I would have most probably bought the Z5 (with A3 sound).
So please do not consider me buying the Z5 an indication that I think it is the better sounding earphone. They sound very similar. The one argument I can make for the Z5's sound (besides the more tactile bass) is that the slightly subdued treble makes it a bit more of an all rounder if you are listening to more hotly mastered recordings in noisy environments.
Most importantly, both isolate better than the H3 thanks to the removal(?) of the ports.
I think this is the most mature iteration of the Sony house sound yet, which is a good sign considering the lacklustre sound of the MDR-Z7 and the somewhat messy MDR-1A.