nesdev.org

Wow after listening to some of the pokemon tunes with headphones on (good ones) I really noticed how good they are. Anyone else surprised by how much better the tunes sound with proper gear?

Definitely - the difference isn't much on GB/GBC because the tunes are basically suqare waves with high harmonics - but on the GBA the difference is just unimaginable.

My GBC has really crap sound trough the jack - probably a bad contact or something.

I only figured halfway my Golden Sun 2 play-through I've been missing a good half of the soundtrack not hearing basslines at all - some other games suffer of this too but GS/GS2 is one of the most noticeable.
That being said, the speakers in the GBA are quite noisy as well - it sounds like there is always wind in it.

The gbc is apparently known for having bad sound.

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