Re: METAL sounds needed in old Ibanez
David Locher's advice on narrowing down where to start is sound.
On pickups, if you want a gonzo metal pickup, it's hard to do better than the PATB-2 Parallel Axis Distortion. It's hot, compressed, but weirdly touch sensitive. Dynamics show up more as alterations of tone than volume. Harmonics just scream out, and ridiculous sustain. A TB-6 Distortion isn't quite as hot or compressed, but has a harsher bite. Black Winter is a more neutral pickup, less harsh and far more dynamic and flexible. A TB-5 Custom is a pretty killer all-around pickup, too. Great chunky rhythm tones for rock and metal. Alternative 8 is another great punchy high gainer.
(Note I'm only recommending Trembuckers, those are the correct ones to slot if you want them to line up with your strings perfectly in a Fender or Floyd bridge, or most modern Gibsons, for that matter.)
For an outlier, a P-Rails Bridge is probably the closest Seymour Duncan to the Air Zone Page Hamilton is using now. And it's hugely more flexible with ability to split to a P-90, a rail single coil, or a parallel mode that is more like a PAF (though some compare parallel more to a Filter'Tron).
As far as pickup vs amp, the amp is the most likely culprit for the bottom dropping out as the volume goes up. If it has no guts when you turn it up, that's likely because it takes a lot more power to produce low frequencies. And the power amp and speaker just aren't up to it. If that's what is going on, you may do better rolling back the treble and presence rather than trying to boost the low frequencies. Too much gain can make things muddy, particularly as volume goes up. JB_From_Hell's advice about where to start tweaking sounds like a great place to start.
If you do decide to go after a different amp, there's no shortage of choices. Save up and search for a VHT or Fryette Pitbull [Page Hamilton used a VHT Pitbull Ultralead, I'm not sure which Fryette is closest to that, one problem is neither company makes them anymore...]. Or get a modeller and search for an amp model that approximates it--Which would be easier if anyone modelled the Pitbull. Sad that the amp disappeared, it was a beast. Megadeth got some of their nastiest thrash tones when Mustaine was using VHT amps. He didn't like it for other riffing or lead tones, and didn't want the complexity of switching amps constantly, so moved back to Marshall power amps.
There's tons of modelling options for high gainers, even if none is precisely a Pitbull. Line 6 Helix or Fractal Axe-Fx are great platforms with tons of amp models to get lost in. PRS Archon (Archetype Lead on Helix) might be a good one, for a dark but punchy huge amp. ENGLs, Mesa Mk IIC++, Mesa Mk IV, Soldano SLO, Recto, 5150, and on Line 6 Badonk and Doom.
One problem of modellers is their quality of sound is limited by your playback setup. A great modeller into a crappy monitor or bad headphones wont have any more chance of sounding good than a great tube amp into an ill-matched guitar cab.
There are plenty of cheap tube heads, but I've never been happy with them at practice volumes, and don't like to recommend something where you need an attenuator. Some people just prefer a tube rig, even with the bulk, maintenance and volume hassles.
Cheapest way to figure out what you like is to try everything you can at local stores, if that's an option. Else, it's off to youtube and trying to guess how much of what the amp sounds like in the video is the guitar, stompbox, microphone, post-processing, as opposed to the amp and speakers/cabinet...
