Rockbox is a set of firmware alterations for a limited number of players. These modifications to the firmware enable changes in the user interface, remove limitations in capacity, allow for larger capacity batteries to be fully utilised etc. Manufacturers tend to frown on these kind of things as it enables people to extend the use of their player when otherwise they might have bought another one. The older iPod Classics are supported, the later ones not so although I think people have been successful, various Cowon models are also supported. There is a risk involved with flashing an unapproved (by the original manufacturer) firmware. If something goes wrong the player may become 'bricked' which means it won't power up or charge because the firmware has gone corrupt or is doing something that causes an error or loop. Sound signature is just the way a player presents the music due to what it is and can be adjusted to a certain extent by equalisation. So a player might have supressed bass, colourful midrange and extra treble. Players like Cowons enable you to adjust these characteristics comprehensively. So if an artist didn't record much bass you could increase the bass amplification to make the track sound 'better'. Usually this distortion of the original signal actually reduces the overall sound quality. Modern players however have sophisticated equalisation tools that minimise this distortion. There are those who believe you can make players sound like different ones by equalisation but not everyone agrees with this.
I just wanna buy something that sounds better than a smartphone
Most dedicated music players will be an improvement although 'phones like the iPhone have good sound quality. You need to decide whether you want to adjust the sound and how important useability is to you. Hi-fi quality players are expensive, use their batteries hard which means short battery life due to all the microchips inside. Mainstream players have better user interfaces, have extras like video playback, games etc. they have longer battery lifes which is convenient but may compromise sound quality somewhat. They also have good parts supplies and support and are less expensive to buy.
Hi-fi - Colorfly C3/C4, HiFiman 601/801, Teclast T51(not available new), Astell & Kern AK100
Mainstream - iPod Classic 6/7gen, Sony models, Samsung P2/P3(not available new) Cowon S9/J3/X7(not quite mainstream S9/J3 not available new)
a player I quite like is the iPod Video (Classic 5G) it is not a no compromise hi-fi player but has a good interface is readlily available used, is easy to repair, sounds good with both IEMs/Full size headphones, has high capacity (30/80GB), can be modified and fairly easily repaired and is durable. With a set of Shure SE425s for portable use I think most would be pleased. You can even transplant the HDD with an SSD.