Gary Moore is standing at a 45 degree angle to his tube amp with it cranked like Alabaster talked about. A studio trick is that you can use anything from a wedge monitor to studio monitors or house stereo speakers to get feedback with some gain going. Most of the examples in the albums you linked are tube amps howling. Gary Moores guitar is using a 60 year old les paul burst named Greeny after the original owner, Peter Green. So he is using the guitar Greeny, which is an old les paul that had its pickups botched in the factory with a reversed magnet and rewired incorrectly (or perfectly, depending on your perspective. To the lobsters in the Titanic that iceberg was a miracle) to make it the only les paul on the planet that sounds like it does when out of phase.
The guitar is now owned by Kirk Hammet, and is a 59 burst. It was originally purchased by Peter Green when he replaced Clapton in the Bluebreakers. And was Moores go to instrument. I mention it because getting Gary Moores sound wont happen exactly without that guitar. Also as a 60 year old piece of legendary gear worth 6 figures, it plays metal every night now on stage with Metallica. There are a ton of videos and info about the guitar, Here is a mini doc from youtube about it:
So anyway, you can still get close with a real tube amp. The old school way to do this is face your amp so the pickups will send a signal to feedback to the amp, then move to the sweet spot which is usually like 3 to 4 feet away from the cabinet usually somewhere around a 45 degree angle. But honestly 100 watt tube amps at full volume will melt you face, kill your dog, and piss off your neighbors, The tech options suggested will work, as will smaller tube amps. For a metal sound I like Mesa Boogie triple rectifier, but again those are 100 watt tube amps made to push air through an auditorium, Mesa makes smaller really amazing tube amps, here is a link to what they sound like (He has a noise gate in his signal chain to stop feedback) But these amps do naturally feedback very nicely with every kind of guitar.