I guess you tried this - From the manual:
DT Amplifier Factory Reset
As mentioned in the above sections, when customizing any settings via MIDI, your changes remain “saved” internally on the DT amplifier. This includes changes made to the preamp and power amp per Voicing switch value, Reverb and more. To return all settings back their factory default state, you’ll need to perform a “Reset” of the DT amp. To perform the Reset, start with the DT amplifier powered “Off,” then simultaneously hold the Voicing toggle switch DOWN and the Pentode/Triode toggle switch UP while powering the amp “On.” Continue to hold these toggle switches in this position until you see the Voicing “I” indicator light up (this takes about 15 seconds). The DT factory default settings are then fully restored and your DT amp is ready to play.
https://line6.com/media/dt50/DT_MIDIGuide_v2_0_RevA_English.pdf