Custom moulded IEMs have a number of benefits. The most significant is the level of noise isolation they offer. Unique Melody quotes an environmental noise reduction of -25dB and we wouldn't doubt that. By cutting down environmental noise it creates a perfect soundstage for listening to music at safer volume levels, even in noisy environments. A knock-on bonus is custom moulded monitors work extremely well as earplugs.
The downside for Unique Melody though, is the choice of a hard shell. This route is common to most custom IEM makers, but they all exhibit the same flaw: the shape of ear canals change. This happens naturally with ageing (custom moulds will need to be changed every 5-6 years and Unique Melody offers a £175 re-shelling service), but it also happens with natural movement – such as when opening your mouth. With a hard shell there is no give so the seal is easily broken. It also means they are more tiring to wear over long periods.
Consequently the Aeros face a significant problem, namely the ACS T2. Coming in at a roughly similar RRP (£500), they are cast in medical grade silicon which makes them flexible – at once resulting in a more consistent seal and flush finish so they are far more comfortable to wear. This results in a better soundstage and the T2's dual drivers require little amplification meaning they actually sound far more powerful then the triple driver Aeros. ACS' choice of sound signature is more flexible responding well to virtually any kind of music and projecting greater emotion, even if they may not technically deliver the same level of detail.
The Unique Melody Aero presents quite a problem. In the plus column they are well made, designs are completely bespoke and, with the right kinds of music, they sound fantastic. Know what you're buying and it is hard to imagine many disappointed customers.
The negative column is more problematic. The hard shell is an inherent problem for any company that chooses it as it affects both seal and comfort. The emphasis on accuracy strips emotion from bass heavy tracks and fiddling with an equaliser isn't an ideal solution. Meanwhile you'll need an amp to get the best out of the Aeros on a mobile device.
Tally both columns and ACS' T2 remains the circa £500 custom IEM to beat.