There's a considerable difference in sound between the different models. You may have heard of the ProSound modification, which bypasses the Gameboy amplifier. (Good when connecting to an external amplifier/computer soundcard but not when listening in headphones) The modification was originally invented for GBC with its crappy headphone output, but was later adapted to DMG as well, where it gives a lower noise level, but doesn't particularly change overall frequency response.
Here's a handy comparison between different GB models:
http://www.herbertweixelbaum.com/comparison.htm