The original screen protector is a bit too soft to last long, not made for everyday use.
I bought a Folix FX-Clear Screen Protector, cheap with great transparency and a hardcoat to resist abuse. Folix is a german brand which offer custom-cut films for almost any player. They also provide other model with different finish: mat finish, glossy with anti-reflective coating, mirror finish...
Next thing: if you wanna buy a case for your X7, please do yourself a favor: DO NOT buy the official Cowon leather case!!! I bought it because the official leather case for my Cowon D2 was really topnotch, but for the X7, it really sucks because it does not protect the sides of the player, which is precisely what should be protected in priority during a fall...
Next advice: custom UIs! Unless you have small fingers and like the base UI, install alternative UIs like Leaf, Sense, Lynx. They will make your X7 way more intuitive and practical to use (larger buttons, easy search, nice covers).
For sound effects, leave everything flat and start playing with the BBE effect first, it should "open" the sound on most IEMs, unless your headphones/IEMs already have a very large soundstage from the start.
Once you're satisfied with the BBE setting, start playing the 5-band equalizer with finesse to fit your taste.
You could also try the presets, but I find them too colored.
That should be it for the main perks.