I doubt you'll damage them with the internal amps, remember that the audio system is based on 16-bit 44.1 Khz PCM digital audio, it has a mathematically limited dynamic range, some tracks sound louder because they have been compressed and carry a higher average energy level (loudness), measured by RMS, peak level cannot exceed the systems limits.
Crank 'em up, if it sounds bad, turn 'em down till it sounds good again. Distortion (square wave) in and of itself won't damage speakers, exessive amplitude will (big amps and little speakers).
Better still, get a good pair of self powered monitors and really rock out.😀 😀
(Internal mac speakers generally suck)