Adrian the answer to both of those, is you can do that. You don’t need to change the size of the cinema 10 though. I would just make an angle bracket and angle it that way. That is going to be the easiest way and it will also ensure you wouldn’t need to reposition the ports. If you make the bottom much smaller, you will want to move the prots to give them ample room to breathe.
As far as the Soundstage 15, I am guessing you are just hoping to save the ports. You could even place the ports up higher (between the waveguide and the woofer – if you can get them to fit well) and then just deepen the box. THis would allow you to cut the bottom part off below the woofer.
