Of course, the Mastering Engineer strives to control the sound to be processed as loud as possible.
In many cases, the heavy metal mixdowns that are sent to me for mastering are too bassy and muffled. This makes a muddy sound, which can be remedied by a few handles.
Loudness is an important issue in the mastering process. Since loudness and dynamics in mastering get in the way, a little sensitivity is needed here.
Which speakers you choose in a studio is primarily a matter of taste. The ear of the mastering engineer will get used to a pair of good speakers over time. In some studios you also use several different speakers.
The acoustics of the room play a major role in mastering. However, you can achieve a lot by Do it Yourself, if you don't have a big budget. But in this case you should inform yourself in advance...
Rock and all its beautiful derivatives such as hard rock, metal and crossover have one thing in common: they base on wide, fat guitars, powerful basses and a voice that is to be integrated neither too quiet nor too loud into the mix.