Best Buy: Behringer POWERPLAY HA8000 V2 8-Channel High-Power Headphones Mixing and Distribution Amplifier Stainless steel HA8000V2
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Behringer HA400 4-Channel Stereo Headphone Amp with Audio-Technica ATH-M30x Professional Headphones + 5-Pack, Gold Plated 6.3mm 1/4 inch to 3.5 mm Male to Female Stereo Adapter - Walmart.com