Note I read one online review of the HPO that said "that only about a quarter turn of the volume knob is needed to achieve ear-shattering volumes" which is why I'm hoping someone on this forum has real world experience with either of these 2 modules and can comment.
To my knowledge, the achievable volume in headphones depends on the relation between output impedance of the headphone amplifiers and input impedance of the headphones themselves. Low impedance headphones will allow for more current to flow and the volume to be louder, vice versa with high impedance headphones. Check the manual/specs of both the module and your headphone set (and ideally the headphones of the user who complained) and decide accordingly.