Sounds to me like the mics or analog inputs are overloading. Given the UA5's analog inputs generally do just fine with other mics and loud sources, I'd suspect the 9v powered mics.
Do you think upping the rolloff from 95 Hz would help or just give a more hollow sounding recording?
If it is, in fact, the mics overloading, then the signal is distorted prior to hitting the HPF. The HPF (bass rolloff) may help minimize the distortion, but it won't remove it (unless you set the HPF so high that it completely rolls off all the frequencies impacted by the distortion, in which case the recording won't be listenable).
So what would you suggest from here? I’ve got a phantom power option, do you think that is the route to go? What would running a battery box with phantom power do if anything?
I've ran this exact setup 3 times prior to this show and had no problems at all.
My 2nd night tape has even more bass, I ran from the 2nd row and my levels were as low as they could go on the UA5 and I was still very close to clipping and sometimes did clip. There was also another guy running on my stand who ran:
SP-CMC-2A > batt box (@95khz) > jb3
He had no issues with bass at all, and his levels never peaked. Oh the joys of taping…