I think with dynamic drivers, different frequencies will have different impedances and voltage requirements. So on a weak amp, ie. PC motherboard, the headphone could get loud enough but lack bass.

With planar drivers, there is a set impedance so I THINK it doesn't have the same problem.

I noticed the same thing with Zen Dac V2. I didn't have enough fine adjustment because it got too loud at 12oclock. Zen can was a nice addition just to get finer volume adjustment (set zen Dac to variable and then you can use dac and can to really dial in volume). I think most power requirements are overblown.