At some stage after the conclusion of the tour to support the Black Album Lars Ulrich shed two toms from his kit. I'm referring to the toms at chest level not the two floor toms which I think he's retained.
The question is how can you do this and still sound the same? Surely there are pre-existing songs that demand the use of these (now missing) toms?
If he went from six to four toms, the simple answer might be that few fills required all six toms, and those that did, didn't utilize them in a significant way. In metal, more is more and that goes double for drummers. Not many drummers with more than 3 or 4 toms utilize all their toms in my experience. Perhaps L. Ulrich came to the same conclusion.
In contrast to metal drummers, jazz drummers have quite stripped down sets, and manage to get a lot of different sounds out of their kits. Four toms are not stripped down (unless you're Neil Peart or Terry Bozzio), and that is likely the reason the shedding is not very noticeable.
Very simply, you can't. Unless those two toms were not being used in the first place, which may very well have been Lars Ulrich's reason for shedding them.
But more realistically speaking, it's possible that he simply wanted to create a different sound. Many musicians agree that sometimes you can make your music better by limiting your resources. For instance, arrangers decide which instruments they'll use before starting to arrange a song, and then they'll adapt the song to this limitation they have imposed on themselves.
In drumming, it can be similar. I'm guessing that Lars Ulrich wanted to experiment with a completely different sound, by using a more limited drum kit. Of course, he'll have had to adapt his parts in all songs so that they'd work this way.
If you'd like to hear to what extent you can make a drum kit sound different by removing pieces, I suggest you have a listen to Alt-J's album, called An Awesome Wave. The drummer doesn't have a single cymbal other than the hi-hat, and also makes very limited use of his bass drum. In my personal opinion, it sounds very unique.
