when i look at DAP/ AMP specifications, there are two parameters i do not clearly understand.
Output level: 2.0 Vrms
Output power:30mW@300ohm
Appreciated if anyone can explain these parameters to me. what does these mean in terms of driving power? and how are they different?
thank you 
Output level is the max voltage the amp provides at very high impedance. It is fairly easy for amps to handle high impedance headphones since the needed current is low.
For low impedance however, the amp struggles to provide the relatively high current. So it lowers the voltage, too.
That’s typically the case with portable amps trying to handle planar headphones.
So the output power depends on the impedance since the current that the headphones need increases when the impedance lowers. The higher the impedance, the lower the needed current.
Output level at 2 Vrms is enough imo. You should pay attention to the output power and also your headphones’ impedances and sensitivities.
What I said is according to these formulas:
Output power:
P=V.I
P=V^2/R
Also:
R=V/I
Depends on what they’re specifying. Output level of 2 volts may refer to line out if the device has line outs. 2 volts is the standard SE line voltage for audio devices. Whereas output power is the power available to drive HPs/IEMs.