The DSOQuad HWv2.6 need to upgrade the app/sys/FPGA as the way in the manual because of the BIOS that embeded in the STM32 when Quad was produced . So, DFU-util or dfuse can not be used unless you embeded the related BIOS to the STM32.
as on the Mac, it will be grateful for you sharing your experience that we really lacked .I expececd your detailed introduction