High-resolution music players have gotten a lot of press over the last year or two, but they aren't new. Hifiman started selling high-res portable, and then Astell & Kern, Cowon, FiiO, Sony -- and just recently Pono piled on. I've tested or heard all of them, and I think they offer significant sound improvement over your smart phone.
The catch: you need a great set of headphones to fully appreciate what a high-res music player can do. So before you invest in an überplayer, buy a really decent set of headphones, and they will make a bigger, more noticeable difference in sound quality than a high-res music player. Frankly, you'll be throwing your money away if you spring for the player, and pair it with just a so-so set of headphones.
The better the headphone, the more detail and resolution it can retrieve from the sound of your music. As for high-res music files of 48 kHz/24 bit or better, it's a safe bet you don't have all that many in your collection, so the good news is a great set of headphones will make standard resolution (CD quality) FLAC or Apple Lossless files sound better than ever.
Just because a file is high res doesn't automatically mean it sounds different or better than standard resolution files. The quality of the original recording comes first, if it's great, it can sound spectacular even as a 320 MP3 with high-res players, with a set of killer headphones. Then stepping up to a high-res file and player will make a difference worth hearing.
As for headphones that let you hear high-resolution files more clearly I'd start with thein-ears, or the full-size headphones.
The($99 in the US, £99 in the UK, AU$129 in Australia) is worth considering, and the Sony NWZ-A17 Walkman ($300, £180, AU$325) is also pretty terrific.
As for ultra high-end models, the Hifiman HM-901 ($1,000) and Cowon Plenue 1 ($1,249, £749) are the best-sounding players I've heard so far, teamed with the better AKG, Audeze, Beyerdynamic Grado, Hifiman, JH Audio, Oppo and Sennheiser headphones. Start with one of those headphones, then move up to the best player you can afford.($1,199, £949, AU$1,599) sounds good, but the