I really can't stand it when when people post in these threads solely to make fun of styles of music they don't like. Honestly, I didn't like any of the clips posted that show what an Alembic is "supposed to sound like". In fact, for the most part (with the exception of Entwhistle's clip) I dislike the styles of music. With that said, I'm not going to knock what those guys are doing purely because it's not my thing. I'm not some raging metalhead -- I listen to a huge range of music -- but I think metal certainly has it's place in the musical world.
To those who feel the the need to make fun of sepultura because they think thrash isn't music... grow up, get a life, post elsewhere, and stop trolling in these threads if your only intent is to bash on something to make yourself feel good. Sepultura have probably had a lot more musical success than you ever will (and no, it's not all "marketing", their early albums were hugely influential and they're one of if not THE most successful acts to come out of Brazil), they're a hard working band, and for those that appreciate what they've done, they have some truly classic albums to their name (Arise is fantastic, IMO). Personally I don't care for any of their music from Chaos AD and onward, but their importance in the metal world cannot be underestimated as they were one of the key bands that led to the creation of death metal, even if they soon moved in a different direction. And yes, death metal has it's place too. Metal is music, whether YOU like it or not.
This clip admittedly sounds bad. I've seen other live footage of them where the bass sounds fine, but I don't think he always used an alembic either (though I could be mistaken). But what band doesn't have some bad live footage out there? For a band who has played literally thousands of shows it's inevitable that some of them will have bad live sound. It could very well be user error resulting in the ugly tone, or like others have said, it could have been an ugly DI'd tone that the sound guy didn't know how to work with. Who knows. But to take a conversation on tone and turn it into an attack on musical style and on the musicians themselves, it's not only lame, but it's disrespectful... and there shouldn't be any tolerance for it on the boards, IMHO.
EDIT: I just watched the video of them again. I think it sounds fine, actually. When it's in the mix the bass sounds more or less the way it should... only when solo'd did it sound bad, and that's often what happens when you get a sound that works well within the mix. Obviously it's not a perfect sounding bass tone, but for punked-out thrash it certainly works. And to those that want to knock the band based solely on this performance: that song is NOT indicative of what sepultura sound like. It was written as a throwback to all the hardcore punk they grew up on, and was written as a protest song about the police in brazil. Their earlier sound is typified by a highly technical brand of thrash bordering on death metal at times, and they evolved into a more tribal, stripped down form of modern metal which was very important in defining what would later become nu-metal... which I'll be the first to admit I don't like at all, but hey, a lot of other people did, so props to them.