ADV.Sound M4

'Flat' is a word which had distinct negative connotations in most reviews, implying lack of any real emotion or selling points. However, when talking about smartphone earbuds, 'flat' is just about perfect, referring to the frequency response, so that bass, middle and high frequencies are all reproduced efficiently and accurately. Such is the case with the rather wonderful ADV.SOUND M4 (and yes, the manufacturer name is supposed to be in all caps, with a period in the middle!), more accurately described as 'in-ear monitors' - with no enhancement or tuning beyond what's actually in your music files, they make great reference headphones for a musician, for example. The ace in the hole turns out to be the supplied 'Comply' memory foam tips though, which are stunningly innovative.