I'm yet to find any genre that doesn't sound its best from the K10. Granted, the hardest stuff in my library is Combichrist and Rammstein, but the K10 has phenomenal speed, resolution and punch. Should be a winner with metal!
May I humbly suggest some metal for you? Most of these are available to listen for free on youtube(full albums), but of course you can purchase flac versions on bandcamp. The first two albums are softer atmospheric metal, good for someone just getting into metal. The fourth album is a must-listen imo, well they pretty much all are actually. I listen to insane amounts of metal and these are at the top of my list so you can be assured they are all top shelf.
The Ocean – Pelagial(Progressive/Atmospheric Sludge)
Caladan Brood – Echoes of Battle(Epic/Atmospheric Black)
SOAR – Roots(Atmospheric Folk Metal, capable of long but not repetitive compositions)
Xanthochroid – Blessed He With Boils(Best Epic metal band on earth)
Hoth – Oathbreaker(melodic death album about darth vader)
Be’Lakor – Stones Reach (Melodic Death)
Woods of Ypres – Grey Skies & Electric Light(Doom)
Insomnium – Shadows of the Dying Sun (Melodic Death)
Tryptikon – Melena Chasmata (Doom)
Panopticon – Roads to the North(Bluegrass fused with Blackmetal)
Intestine Baalism – Banquet In Darkness (Brutal/Melodic Death Metal)
Bolzer – Aura. 2 man duo, lead singer plays 10 string guitar(Blackend Death/Experimental)
Orphans of Dusk – Revenant (Doom)
Ne Obliviscaris – Portal of I(Progressive)
Heaven Shall Burn – Antigone (Melodic Death)
Empyrium – Weiland (slow atmospheric metal)
Vektor – Black Future (Progressive Thrash)
Black Fast – Starving Out the Light(Melodic Death)
Benighted – Asylum Cave (Blackened Death/Grindcore)
Thy Catalfaque – Rosa Hasa Radio(Avant Garde)
Arbrynth – Abrynth (Melodic Death/Progressive Folk) Sent from my iPhone
Cannabal Corpse – Anything (Death)
Metal is such an expansive genre...once you get into metal it's like spelunking in a never ending chasm, you are constantly going down new passages and sometimes you find hidden treasures.
Check out this badass interactive map of metal:
http://www.mapofmetal.com (looks better on pc, too big for mobile imo).