HD600 / HD650 CNC Copper Mass loading mod DIY kit – Custom Cans Shop
OG models (JAR580/JAR600/JAR650)
Jupiter Audio Research "J-Mod" (HD650 mod) | Headphone Reviews and Discussion - Head-Fi.org
Sennheiser HD650 / HD600 mods - Custom Cans Shop
DIY Audio Electronics from Zynsonix.com: Cable and Mods for Sennheiser Massdrop HD6XX
NwAvGuy: Sennheiser HD 650
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...]
HD650 KISS mod | Super Best Audio Friends
Sennheiser HD 650 Mod Compilation | Page 4 | Headphone Reviews and Discussion - Head-Fi.org
HD6XX KISS Mod Impressions : r/headphones
Jupiter Audio Research "J-Mod" (HD650 mod) | Headphone Reviews and Discussion - Head-Fi.org