If you don’t know anything at all about electric guitar pickups, I recommend you read something like this first, for some basic background: http://www.dawsons.co.uk/blog/main-electric-guitar-pickup-types
Let’s talk about pickups for heavy music. The first thought that a lot of people have is more gain=more heavy. High-gain, high-output pickups have their place, but they are also not the end-all, be-all of heavy guitar sounds.
To demonstrate my point, I made a short video (https://www.youtube.com/watch?v=rRY9w5mtRmQ&feature=youtu.be) comparing the pickups in two of my guitars: an Ibanez DTT700 and a RGT42DXFX (inspiring names, I know). The DTT is a Destroyer with DiMarzio D Activator X pickups, and the RGT is a super strat with Ibanez Infinity pickups. Though they are shaped differently, and the Destroyer weighs about eighteen million pounds, the guitars are actually rather similar in most important ways. They are both neck-thru instruments with mahogany body wings, passive electronics and a hardtail bridge. In terms of sound, the primary difference between these two guitars is their pickups.
The differences are pretty audible, and what you like best is really up to your own taste and style of music you want to play, but let’s talk about what we’re hearing a little bit. Compared to the INF pickups, the D Activator Xs are “tighter” sounding (which, at least here, to my ears, is them picking up more treble and a little less of the far low end). The other main sonic difference is the dynamic range possible with each of these pickups. The DiMarzio set is more compressed, meaning that there is less of a difference between maximum and minimum output. They also are generally louder than the Ibanez set. This is especially clear in the last two clips, where I play a D power chord very softly and then very loudly.
You can argue for hours (as they do on reddit/r/guitar) over why or why not compression is good or terrible and how vintage things are more “open” and less “shitty” or whatever, but the truth is that it’s your sound, so use what you like. I love the way both guitars sound. For my part, I find a lower output pickup to be better for more dynamic, textured and layered music (If you’re a fan of Tool, you might go for something with less output, as an example). High-output pickups are great for when you need your guitar to be a sledgehammer, or if you’re mostly concerned about playing shreddy leads (where compression is a must). That’s all pretty obvious, but it’s important to think about what you’re trying to accomplish before you just buy the baddest, most mother-fuckin’ pickups you can find because you want to crush.
“Ok,” you say. “But how in the Satan do I tell which ones to get?? What the fuck is an Alnico?”
Valid questions! Well, there are active and passive pickups. Passive pickups tend to be more colored (that is, not having a flat frequency response) than active pickups, and active pickups are usually more compressed and higher output than passives. Personally, I can’t stand playing active pickups, but lots and lots of metal guitarists love them. Don’t let me stand in your way if you love a set of EMG 81/85s. Active pickups use weaker windings and magnets and then use a battery-powered preamp to boost the signal from your guitar. Passive pickups do without the preamp, and so must generate high output from lots of wire and powerful magnets. What that means is that passive pickups deliver a pretty colored signal and vary a lot between models. Active pickups deliver a very loud, relatively uncolored signal. There are also a lot fewer models to choose from.
If you go the passive route, you have three main magnet choices, Alnico ii, Alnico V, and ceramic. Alnico ii magnets are usually only used in vintage-voiced pickups, and tend not to be as loud. Alnico V is pretty standard, and pickups using those magnets range from pretty mellow to crazy powerful. Ceramic magnets are generally the loudest, and can be “brittle” or “harsh” sounding, but they can also be very powerful and clear.
In my video, the INF 2 pickup has an alnico V magnet, and the D-Activator X has a ceramic magnet. The INF 2 also has traditional pole pieces, whereas the DiMarzio pickup has huge blade-style pieces. See the pictures!