From what
@jessnie is saying it seems more brightness is not something he/she wanted
Lionheart slightly boosts mid-hi which may result in a tiny bit brighter sound than stock, but it is not something disturbing. From my own trial of the Lionheart with the U18, I unfortunately found the pairing brought little to none sonical improvements that I decided to move on. On the other hand, I found the Leonidas smoothens the frequency response (without boosting the mid-hi or presence range) such that you may enjoy the monstrous high and ultra high extensions of U18 with zero fatigue.
Caveat: cables are much more niche than the earphones themselves. Don't trust too much what others say on them (me included). Some very seasoned head-fiers still maintain cables make no change to the sound quality, and while I would not take that extreme view, I do maintain that COSTLY cables with X mixed with Y coated with Z precious metals does NOT mean the sound will become better or, well, more precious lol. Must try it out yourself with your own earphones to find out what works for you.