"The Solo SRG II headphone amplifier has two line level inputs and a front panel input selector with centre off. It is housed in a strong anodized aluminium eurocard style enclosure. The volume control also acts as sensitivity adjuster and therefore the full rotation can be used . The Solo SRG II headphone amplifier is suitable for use with headphone impedances from 16 Ohms to 600 Ohms."   I bought this amp unheard based on the rave reviews from English hi-fi magazines and it did not disappoint. It runs my Grado 325i's as well as my HD650's...