ok. i've completed the mod and i've done some recordings.
Here some useless nice graphics, lol:
GBA SP Stock:
GBA SP Prosound:
DMG Prosound: (I have resized this one, it is a lot louder than the gba)
The output of the prosounded gba is much louder than the stock one.
I haven't heard any hiss/hum fixing, but the nice thing is that the hum remains at the same volume in both models.
..so the louder output of the modded one covers the hum very well.
The DMG (with prosound) still remains the best option for hiss/hum and volume.
It's louder and it has no hum.. but it has some more hiss and less basses.
So this mod doesn't change too much the output of the gba (except the dB gain), but surelly it's awesome to have a built-in jack output on a GBA SP (not in my case, lol).
Yay. if you want to hear my recordings, here the link.