As with most marimba One Mallets tend to be, I believe it is best treated as one level softer than what is written (so this would be a "Medium" by most other brands' metrics...)
The sound is very dark, and they do not project enough for ensemble pieces, however the quality is top notch and the mallet itself is very well made - again as expected of Marimba One. It is a good (even great!) mallet for medium intensity pieces, and even works softer solos on Vibraphone (I played Ysaye's "Malinconia" - originally for violin and transcribed for vibraphone - with these, and was rather pleased).
For more intense solos and ensemble works I would highly recommend looking at either hardness 1 of this series, or potentially even harder models by other companies as these mallets lack the hardness to truly reach a convincing forte... Still, these are good general mallets and offer fantastic value for money.