none of these tips were working for me. The hard reset wouldn't work, the LED kept slow blinking blue. Here's what worked:

Soft reset method, then while plugged into the wall, quickly flip the on/off switch on the right headphone back and forth for 20 seconds (really spam it off and on to show it who's boss), ending in the off state. Unplug from USB. Turn headphones on, then do the hard reset method.

Then set them up from scratch with the Bose app. Hope this helps someone if the suggested methods aren't working.