Tia drivers tend to add treble quantity and extension to the sound signature. As a result, you'll get massive boosts in detail retrieval, clarity, air, transient speed and stage openness. High treble extension translates to a more stable stage for more reliable imaging and prevents mid-bass energy from congesting the image. I can only think of a few downsides as far as Tia technology is concerned. The pros associated with upper-treble energy may come at the cost of warmth and naturalness sometimes, but thankfully 64Audio haven't crossed that line and made an offensively bright IEM yet - nor do I ever wish it upon them to.
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have talked to some treble-sensitive people who take issue with the quantity of sparkle that the new Tia-equipped IEMs have - particularly the Tia Fourte (as it's currently one of the most highly-documented of them all) - but it's definitely more subjective than objective, and their complaints are relatively minor.
My dad owns an A3 which I've heard via force-fit multiple times and I think the A6t is a great choice to upgrade to. The U6t is neutral-bright in tone based on my impressions at CJ SG 2018, so it's definitely more akin tonally to the A3 than the old U6, which was more organic and full-sounding. The main aspect you'd be missing out on from the A18 (apart from technical performance, obviously) would be mid-bass energy. The A18 is more energetic down-low, but its swift decay still maintains an excellently clean stage. Though, if you don't mind sticking with the low-end quantity of the A3, then the A6t is definitely worth considering.