Brass is powerful, and requires a powerful microphone. A good brass mic emphasizes the character and depth of the sound without being abrasive or harsh, and can withstand very high SPLs without faltering.
"...in comparison with a number of other well‑known ribbon mics in the tests at AIR, the RNR1 presented a real sense of 'air' and definition. At the same time, though, the character of the sound was unmistakably 'ribbon' — warm and comfortable to listen to, with a smooth, non‑fatiguing character."
- Paul White, Sound On Sound (full review)