Heavy Metal has a lot of energy in all of the frequency bands. Because of this, a proper Harman curve is very important. You can experiment with the so called BBC dip, which basically means a recession in frequency response somewhere around 3khz and it will be most noticeable with metal music. The guitars are creating a lot of overtones.
That said, there is a lot of very badly recorded metal music and also a lot of classic records from the 80s where standards were different. You can enjoy these classic albums for what they are, but really good sounding metal albums are few and far in between.
Midrange, especially upper midrange is most important for metal music. From that perspective, a good speaker with a downward slope can do wonders. However, the "software" so to speak is often lacking.
If you want good sounding metal music, I would get something with a proper response on-axis as well as off-axis such as Neumann, Genelec, Revel. Then you may need to apply room correction to arrive at a proper Harman curve at the listening position. This will greatly improve the quality of any (including metal) music.
In fact, I would start with DRC if you already have somewhat proper speakers.