The cabinet of Acoustic Mass is not Cardboard, it is Medium Density Fiber (MDF). Sticking this is a tricky thing. The best Adhesive for this is Araldite or Fevitite. These comes in two tubes a 75 gms tube should be enough it depends on the area to be applied. I am explaining you the step by step procedure below.
1. Part the broken board or the front panel from the board.
2. Scrap out the loose fiber particles as these will not allow both the pieces to be merged together. Place both the pieces together before applying the adhesive and see that they are fixing firmly each other firmly in perfect level if not scrap out some more fiber.
3. Mix the Resin and Hardner of the adhesive in equal quantity and mix thoroughly (do not interchange the caps of the tubes then you will never be able to remove) and apply on the both the broken piece then stick both of them together. Apply constant force either by placing weight or tying it up are by some suitable method. The curing time for this is 8 hours to achieve full strength. In the same Adhesive quick setting is also available which will get set in 15 mins. time. In case you are confident that you will be able to align the broken joint with in 2-3 mins. then you can chose this.
The important point in this whole process is matching the broken area by suitably scrapping out the loose fibers. Once you do this then this area will never brake again, only the adjacent area may break if mishandled.
If you can post some pictures I can thoroughly explain you the specific process itself.
Do Not try any other adhesive as they cannot ensure firm adherence in MDF