The Arturia reads 15V/3A on the frontplate, so that indicates the demand from the powerbrick. However, it´s only specified to deliver 12V/1.6A on the +12V distribution. That´s just half the amps... is this limited by the power module itself or is the distribution board limited to 1.6A ?
No, this makes perfect sense. Remember, that power supply is also feeding a -12V rail plus the 5V rail. Each 12V rail has a possible maximum load of 1.6A, and the 5V rail can handle 900 mA.
However, if you're exceeding the current limits for your P/S, you're screwed. Your only solution is to add more current capacity via a second P/S, or to get a case that has a beefier power supply. You cannot exceed maximum rail loads without incurring some sort of problem...either damage to P/S components over time, or an outright P/S failure all at once. And depending on the power supply design, a "failure" might be more catastrophic than just popping the P/S itself.
NEVER come close to your power rail current limits. The proper rule of thumb is to exceed your rail load by at least 25%; if you have a load of 1A on your +12V rail, the power supply needs to be able to supply 1.25A. This is because, most of the time, manufacturers list the current load during operation and this doesn't reflect any inrush current loads that might happen when the synth is powered up. But even though inrush is only a problem in the first 100 or so msec when a circuit is powered up, that's all it takes for a catastrophic P/S failure. Also, the higher you can spec your P/S, the better, since a power supply under a heavy load is going to have more component issues due to thermal factors than one that's loafing along. A supply that can output 3A per rail or better will last a lot longer under a 1.5A load than one that only outputs 1.6.