Hi gtxkid,
I believe a limiter circuit should be in headphones used on any of the MXT models.
MXTs do not have a control knob labeled as volume control. However, as GAIN is turned up, output volume to headphones becomes louder and at high GAIN settings can be VERY loud. When hunting for deep targets, operators often turn the GAIN up as high as possible and the headphone volume up high to cause deep weak signals to become loud enough to recognize. With the GAIN and headphone volumes high, shallow target signals can have painful volume. It is at high GAIN settings that a limiter circuit can save you some pain and shield you from long term hearing loss. Good quality headphones with the limiter circuit activated will reduce the volume of loud signals to acceptable levels without effecting medium to low volume signals.
I have owned and used a few detector models. The only models I am aware of that absolutely need a limiter circuit in the headphones are the MXTs. I certainly have not used every detector out there so there may be others that also need the limiters.
Good luck
Bob