What is the frequency response of your UT wireless units, specifically on the lower end? Is it below 100 Hz?
According to the UT User Guide, the UT series frequency response is 20 Hz to 16,000 Hz, +/- 3 dB.
If you want to use the UT system (or any wireless system) for bass guitar, please audition it first using your bass and your amplifier. Wireless systems typically never sound as a good as a cable for bass guitar.