I tried the 215, 425, AONIC3 & 4 & 5 recently, and out of those 5 the AONIC3 to me is my favourite. I had no problem with fit, and listening in the store, the isolation on these also felt great. I remember I could listen to the AONIC3 while panning around and felt immersed undistracted with the sound. It's as if a music is playing while a show reel is going on. Good isolation and fit (at least to my ears). Very straightforward to insert over ear, and lightweight too
I mentioned 'immersion' as a way to measure how good these were, partly due to the fit and isolation, and partly and more largely to me due to the clear neutral and colorless sound that AONIC3 provide. The AO3 to my ears have one of the most tonally accurate mids I have heard among IEMs in this price range (and even higher). Piano sounded clear and spot on, and guitar strings sounded nice and natural without dullness or too much warmth. They sounded gentle and refined with decent resolution trying out from my phone. I'm guessing this would increase higher the better the signal fed. A3 is strongly suited to instruments playing mostly in the mids, and upper-mids area, and to a lesser extent in the lower-mids range. These are great for vocals, and previously mentioned piano and guitars. Though on the downside of sporting only 1 BA, the range or reach on the A3 isn't that great, as can be heard when listening to bass lines, and deep kick-bass or sub-woofer sound. Bass aren't their stronger suit. But if you're more of a midrange purist, these are a great delight and especially at its price. Mids are exceptional and beautiful on these, and I'm liking them even after twice demoing at the store for two hours. Consider the AONIC3 if you're after a midrange specialst. But if you'd like to have more color, bass, expansive wide soundstage then these may not be it. I'd suggest the AONIC4 for the warmer sound, being the warmest in my estimation in the lineup, secondly after it came the 425 for warmth
The AONIC4 and 425 to me have pretty similar tuning idea behind their sound. Both have a pretty good reach down low, and rather hefty punchy bass. The 425 sounded sparklier and is more prone to peakiness in the upper-mids, while the AONIC4 sound more linear and refined with more stable imaging (a bit more resolving). That said the AONIC4 have also a less exciting upper-mids in the lineup, and overall flatter, less exciting (more towards an L-shaped signature). Which may suit those sensitive to hot treble. I dislike the single DD 215 so much, that I only put on for less than 20 seconds. So can't comment too much on that. Only that it sounded blurry and too inaccurate
The AONIC5 to me brought the mids of the AONIC3, and elevated it with increased bass, soundstage size, and resolution. Unfortunately, this also brought a problem of joining the 3 BAs to perform together in harmony. I felt that the mids weren't as pure as the AONIC3, and they were more colored, warmer, and less clear or sparkly. They're also more than twice more expensive, because the A5 have more drivers. More drivers, more materials to assemble, which in turn increased cost of making. So to me the AONIC3 sounds like a good deal in Shure's lineup, considering the cost and SQ you get
