Amazon.com: Sennheiser HD 650 - Audiophile Hi-Res Open Back Dynamic Headphone : Everything Else
Archimago's Musings: MEASUREMENTS: Sennheiser HD650 (~2010-2012) and HD800 (~2010). A comparison of two "Reference Class" headphones, and a look/listen to the "Sennheiser veil / darkness". [Inexpensive 2TB SSD and inflation in consumer electronics...]
Sennheiser HD 650 Open Dynamic Wired Headphones with Adapter | World Wide Stereo
Sennheiser HD650 Review
Sennheiser HD 650 Open-back Audiophile and Reference Headphones | Sweetwater