There were times when I also felt nothing from 4 mg and even 6 mg of clonazepa and form tremendous dosages of all benzos I ever tried. All changes after I tried ketamine for several times. Now even 0.25 mg of clonazepam produces some feelings and 0.5 mg is definitely felt. DXM is related to ketamine as its dirty cousin, both of them are NMDA antagonists. But I am not sure this mechanism is immediate action, I am not sure you can take DXM and benzo and benzo is immediately felt potentiated. Some ways of potentiating benzos is taking the inhibitor of the ferment, which metabolizes this benzo. Lets say, diazepam is potentiated with cimetidine, if clonazepam is also metabolised by CYP2D6 enzyme, this would be the immediate mechanism of prolonging its action, and maximizing Cmax a little bit. I dont remember what enzyme is responsible for clonazepam metabolism.