There are about as many different joint as there are different speaker drivers! some fundamentally better than other and some are just preference! If you were to invert your your double rabbit joint, it would be a little stronger. Since the speaker box will not be seen from the inside and has no conflicting internal parts. I prefer to use and inside glue block that is screwed into the corners (not nailed).

Your idea for corner dowels is similar to "through dovetails" and would look great! you would need a jig with a drill bushing that you could clamp onto the corner of the box, and a very sharp brad point drill. maybe someone with more veneer experiance would have an idea. I would probably make one fixture that was the length of the corner. this would have all the dowel hole in it, and able to take different drill bushings. this way you could drill one series of holes undersize. replace all the drill bushings, and drill your final sized hole. this would give you a clean hole on the final pass. a lot of work huh? cool design! have fun.

Bill