ALO audio Rx MKII Portable Headphone Amplifier - Silver The next generation to the award winning ALO Rx Prescription portable headphone amp. We are proud to bring you the new ALO audio Rx MkII Hi-Lo gain front mounted switch with power to properly drive 600ohm headphones on Hi gain Improved bass response, slam and audio power. lluminated on/off "smart" LED switch, indicating charging and battery status. Made in the USA