I have an original PWD II and decided to try to listen to firmware 2.2.0. I loaded 2.2.0 up and it appeared to be fine. I turned the Unit off, removed the SD card and restarted. Instantly the DAC looked dead. Nothing lit up. I popped the top off and checked continuity of the fuse and the digital board fuse was blown. I replaced the fuse and the same thing happened, it started once but when turned off and restarted it blew the Digital fuse board instantly again.
I replaced the fuse again, inserted a SD card with Firmware 2.4.6 and restarted. It loaded initailized and worked fine. I turned the unit off and restarted, no problem. Did this a third time (cycled off/on) and again no problem.
PWD II (original not upgraded), no bridge, running MacBook Pro into PWD II with USB, PWD II plugged into dedicated Bryston BIT-15 balanced power conditioner.
Glass fuse sounds more extended, crystalline but perhaps a bit more lean and “Hi-fi” sounding when compared to the original fuse that came with the PWD II.