So, I hae a broken pair of DT990's. One of the channels is significantly lower than th'other. Like 18db lower.

First of all, for my edification -- what would cause such a thing? I would have guessed a broken connection, but I can still hear the signal, albeit significantly weakened. A short? An almost broken connection? I rule out the latter, because it does not cut in and out. This is all just theory -- I'm not much of a solderer.

Secondly, who would be a good person to send to fix it, and perhaps to upgrade it at the same time (new cables)?

Sounds like you probably have a bad driver. You have a few options though, send the phones into beyer usa and let them fix it or order the driver yourself and send the new driver and your phones to one of the DIY people and let them do it. Of course you could also have them upgrade the cable at that time, for this time of work I'd recommend Headphile or Enigma Audio.

Yeah that sounds more like a driver issue. Try wiggling your cable at different points. If the sound remains the same it's unlikely that it's an electrical connection problem.