You need to find the max voltage the amp put out and the impedance and the sensitivity of your headphones and the max power it can take..
It's similar to how you calculate for amp requirement for loudspeaker but you don't need to add an extra 3 dB because headphone are sealed to each ear, where stereo there are two speakers and you don't need to worry about distance. (unless you're dealing with something like K1000)
Say Your headphones has sensitivity of 80 dB/1mW. To gain the next 3dB you need to double your power.
That is if you feed it 1mW you get 80 dB.
2mW you get 83 dB
4mW you get 86 dB
8mW you get 89 dB
16mW you get 92 dB.... etc
Headphones with Headphones Amps can all typically reach above 100 + dB, some above 120 dB even. So to get 90 dB shouldn't be a problem as long as your headphone can take the power.
To work out how many Watts your headphone is getting from the amp, you need to get the impedance and the voltage out specs of your amp.
You can use this site like this one below to convert from amplitude to power:Conversion between dBm - dBW - W and dBuV - dBV - V