Although the amp chip in the A55 is same as in some higher end models, the power supply is different and as a result the power output is also different.
You may consider using the A55 with lower impedance headphones. My MDR-1Am2 (16 ohms) sounds great with the A55, I almost never turn the volume up more than 75/120. In fact the stated maximum power output of the A55 is given at 16 ohms. If you don't like the 1A consider the Panasonic RP-HD10: 50mm drivers, 18 ohms, 4Hz-50kHz response. Finally make sure that your A55 is not volume limited. To my knowledge only EU models are, so I personally had to change the region CEW2 to E.