Just tried Deadmau5's Aural Psynapse on my 2011 MBA and at 2/3 to 3/4 volume it's got quite a kick! Really don't want to go much higher than that for an extended period. To answer your question, you will get the volume plus better sound with any of these USB dac/amps:
Low $ to Hi - FiiO E07, FiiO E17, AudioEngine D1, Arcam rPAC, AudioQuest DragonFly. If you're looking for portability only, I'd do the DragonFly with the DragonTail (IMO assuming that you have better luck that I'm having with my Macs) or the FiiO E07.