This kind of music reminds me two monster in the history of rock 'n' roll : The Beatles & Kiss. On our interview, King's X reply on this comment : "There is no Kiss influence, but I understand what you mean : it is the groove part of our music that reminds you Kiss' music style". We couldn't say it any better.
I don't thing we can "classify" this type of music with metal style. It sound so "quiet" and King's X music is a very mix of "Beatles meets Kiss" ... or let's say it this way : it's heavy ("Marsh Mellow Field", "à la" Nirvana, melancholic ("She's Gone Away" & "Charlie Sheen" chorus will remind you the Beatles) and totally timeless ("Julia"). Overall, it is a "quiet music". King's X continue to be innovators and don't follow any trend. That's called : character.