(TrendHunter.com) The Sennheiser HD 400 Pro headphones are an open-back audio accessory for users seeking out a way to partake in mixing and additional post-production work in a precision manner. The headset makes... (TrendHunter.com) The Sennheiser HD 400 Pro headphones are an open-back audio accessory for users seeking out a way to partake in mixing and additional post-production work in a precision manner. The headset makes...