meambobbo's analogy of the 2 treble boosters is quite interesting. That would be how I would put it. Tight spectrum.
The more time i spent with all those different tube amps the more it becomes clear that most high gain amps get their characteric tone just by three basic components:
Pre EQ -> Gain Stage -> Post EQ
Just by experimenting with Pre and Post EQ you can get nearly everty tone out of every amp that creates its sound mainly through the preamp section. Tweaking the Pre EQ has similar effect as using specialized pickups. Modern hi gain amps basically follow the same rules as modern hi gain pickups. The key to a tight sound with high string separation is removing low frequencies before the gain stage. If might seem strange that the method for a punchy low end rhythm tone is actually cutting the bass but once you have learned this lesson it´s much easier to get what you want. Using tube screamers or treble boosters before the amp all follow the same approach, removing low end as as Pre EQ.
After the Gain Stage you can compensate by adding low end again while the sound stays tight.