J'ai envoyé un courriel à Jason Lollar, qui est un peu une sommité en matière de micros. Pour ceux qui lisent l'anglais, voici sa réponse, assez intéressante.
"I could recharge the magnets but it's not very likely that it will make your pickup twice as loud. Usually it will give it a little more treble and dynamics.
I have also seen dirty contacts on the output jack or volume and tone pots, and bad solder joints reduce volume.
To check for an open coil that needs rewound, you need a DCR meter to measure the DC ohms. It should read 6 to 8 K ohms - 6,000 to 8,000 ohms; if it reads something like 200,000 ohms or 2 megohms or higher, it has a break in the coil. It will read extremely high and you'll see the reading go lower and lower over a few minutes until it almost reaches zero: it's an open coil, so it needs rewound.
If one of the magnets are flipped the wrong way, it will cancel the magnetic field to zero, and will dramatically reduce the output. Try taking one magnet and flipping it so the side touching the poles is now the farthest away from the poles and see what it does. This is common with that type of pickup, people take the bottom off and the magnets will fall out. There will be two screws on the bottom of the pickup; loosen them and you can flip the magnet. Be careful not to overtighten them because the tortoise shell bobbin is fragile. If it needs rewound I can do that or if you need a complete new pickup made I can do that too. See if the magnets are flipped or if it has an open coil first."