There have been situations where powering a TGXL using a 13.8 VDC power supply experienced voltage sags resulting in anomalous operation with the TGXL. Operating the TGXL using a 13.8 VDC power supply that is also supplying DC power to other devices, may not be sufficient.
The TGXL has a minimum working input voltage of 15 volts. It can operate on voltages up to 25 VDC. operating the TGXL below the minimum voltage threshold is not recommended.
Several TGXL owners have been using an 18 VDC @ 2 amp power supply with good results. Here is a power supply available from Amazon that is compatible with the TGXL's coaxial power connector: https://www.amazon.com/Adapter-Compatible-Brookstone-Bluetooth-Speakers/dp/B0BLRPQ3YQ/ref=sr_1_7