BuyDig offers the Beyerdynamic Headphone Amplifier, model no. A20, for $499. Coupon code "AMPED" cuts it to $299. With free shipping, that's the lowest total price we could find by $140, although we saw it for $5 less on Black Friday. It features an ALPS potentiometer volume control, high-gain RCA input, hard-wired RCA output, two stereo 1/4" headphone outputs, brushed aluminum housing, and 10Hz to 100kHz frequency response range.