Agree with Lee - nice design. Will do just want you want it to !

Some other minor things to think about:
Z travel is quite long - I guess you need that much to do the speaker cabinets if they are partially assembled? If you want to cut aluminium then you will probably need to raise the job towards the cutter to limit the reach of the tool and keep the stiffness up. Some thick packer plates or wooden blocks should do it but you can add those when you are ready.

At the bottom of the frame I'm not sure I'd do the diamond in the base legs, may be just a diagonal across from one corner to the other, and I'd probably add adjustable feet to level it.

That's it really . . .