Archimago's Musings: MEASUREMENTS / MUSINGS: Headphone impedance, sensitivity, efficiency, and amplifier output impedance. (And a quick thought on Darko's "All You Need Is...")
Amazon.com: beyerdynamic DT 990 PRO 250 ohm Studio Headphones (Ninja Black, Limited Edition) with 4-Channel Headphone Amplifier Bundle (2 Items) : Electronics
6) Beyerdynamic DT-990-PRO-250 Studio Tracking Headphones+Presonus Headphone Amp - Walmart.com
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