Create a shape like on the image below:
Offset it by 0.5units and extrude both to the middle of the base model.
Move the inner part from Top viewport to the right by 0.5units. Now, trim this inner part with outer shell of our base model, and outer part with inner shell of base model.
Next, create Circle and offset it 4 times to inside by 0.5units.
Inner two curves will be projected on outer shell of back surface.
Using split command split the surface with those two projected curves and delete the ring surface and move the circle surface inside by 2 units.
Using blendsrf blend the back surface and that circle surface. In adjust blend bulge settings window, set both ends to minimum (0.25)
Do the same for outer two circles and inner shell of base model… This time, split circle surface move towards inside by 2.5units. The shell thickness is 0.5, remember?
In the center of this little detail, create a circle with diameter of 3 units, and trim the inside. Connect with Loft two edges of both inside and outside shell…
Next, create a rectangle 5.5 units wide and 5.5 units high. Create in all 4corners of that rectangle circles radius of 8 units.
Offset those circles by 0.5 units inside.
Extrude those circles (8 of them) so it fits the images below:
Now, trim off the parts that are outside our base model. Keep in mind, the inner tube should connect to the outside shell of our base model, and outer tube should connect to inner shell.
Now, we can fillet the edges. I used 0.5 units for outside and 1 for inside. You actually do not need to fillet the inside edges, as they really don’t show too much.
Now, we will make a cap for those tubes. I have created the cross section curve and then used sweep2 and made a sweep 2 rail. But you could also have used revolve for this. Then fillet the edges with 0.5 units, and copy 3 times.
Extrude this rectangle with cap set to On:
Make a polyline as on the image below
Extrude that curve, and trim with the extruded rectangle.
Copy that part 3 more times, or just simply use mirror. After that, using trim command, trim off the parts of cut rectangle, and inner shell of base model so you can join all those in one object.
And yey, we are finally finished the base shell and case. Now, we will start doing the metal plate and the speaker itself.