If this occurred wheeling, I highly doubt bleeding the ABS would do anything. It's VERY unlikely that air would get in the system especially near the ABS HCU. I also highly doubt that air got in the system. It's 100% impossible to get air in the system unless the system is somehow opened/breached or if you run out of brake fluid. I would check for mud/debris/crap built up around rotors and backing plates etc. It's strange how sometimes a mechanical problem in your brakes can create a different pedal feel, and not just a hydraulic problem. I also would ensure that you have no fluid leaks anywhere. Check around all bleeders and banjo bolts to ensure there is no chance something snagged and somehow loosened a bleeder or banjo bolt. Lastly I would check to ensure there is no dirt/debris in any of the hardware on the caliper brackets. You're more than welcome to have a dealer bleed the ABS, but I don't think it will help. Also, a booster concern will generally create a hard pedal not a soft one. My gut feeling is that mechanically, something has changed the way the pads connect to the rotors even if it is just dirt/debris.
You could also drive this thing around about 20 miles or so and when you get back home, use a laser thermometer on the rotors to see if one rotor is extremely hotter than the other indicating something wrong with a caliper. This usually won't create a soft pedal, but could be something worth looking at just to eliminate a binding caliper.
