My prosounds in my DMGs are loud on speakers/amps but not with headphones.

Use an altoid tin headphone amplifier if I'm not working with a mixer.

^What all these fine people have said. I have both the regular headphone jack, and an additional external prosound mod. I use the prosound output for live performances/recording and the actual headphone jack for... well, headphones. Haha.

But yeah, if it's an internal mod, it will be a tad bit quieter. Once you connect it to a pre-amp, or a mixer... DAT BASS.

My Audio-Technica ATH-M50 phones sound great through a prosound also.

earbuds work well, headphones much larger than this are a bit troublesome however, as egr said

katsumbhong's advice is spot-on.  the prosound mod is (simplified) a by-pass of the internal headphone amplifier. 

this is the best resource I know of which explores the variations between stock consoles, and prosound modified ones: http://www.herbertweixelbaum.com/comparison.htm

Hook it up to a pre amp. Prosound skips the amplifier in the gameboy meant to boost the signal for headphones.

For headphones yes, the sound will be quieter because the impedance has been changed to work better with mixers, amps, etc.  I've had pretty good luck using earbuds with prosound jack output tho, just depends on the phones you're using.

Hi there, I pro sound modded my gameboy color and the sound in much better but it is quieter. Is this normal?

I did an internal mod that is post volume potentiometer. It uses the standard headphone socket by removing 2 resistors to disconnect normal sound.

Can anyone comment if there gameboy color pro mod is quieter?

If I connected the audio lines before the volume potentiometer would this increase volume?