Small update -- saw same band again tonight, same venue, used different gear. This time I think I actually heard that sound with my ears, though it was somewhat less prominent than in the recording. Last night people onstage did complain about tech issues, so I'm going to listen more carefully to this recording and see if it's present or not. But, I'm at least allowing that maybe this wasn't my equipment. It sounds to me like the kind of DI noise I get if I record an acoustic show and the SBD isn't set up properly/there is a general DI problem.