I have a pair of Sennheiser HD600's (now 20 years old) which I currently use with a Yamaha RX-V373 receiver (silent cinema) for films, and it's excellent for the job. However, I recently got interested in new music again and it just sounded dull and boomy. I thought I'd try the smsl m6, not expecting much, and was pretty shocked by how differently it sounded. All the muddiness melted away. The toggle is easy to use, and it's loud enough for me at 28, with it going up to 60. There's also no background noise. I've connected it via USB to a 15 year old Windows XP computer, with the JRiver Media Center (via asio4all), and it plays back 24/192 files perfectly. I've also played HD files via optical from a Dune media player. Pleasantly surprised. Had expected to return it within the allotted 14 days, decided to keep it within a day.