Christof, are you actually using lock mitre joints for speaker building? Looks interesting. I've only used straight butt joints or on occasion a half lap joint. As long as your cuts are square, just straight glue (clamped and maybe a screw or brad here or there to keep things aligned during glue-up/drying) seems to provide enough strength. Is the benefit of lock mitre for alignment of strength?
Ever use biscuits to help align? If so, ever in MDF? What about fitting braces, do you rabbet them in, or just butt joint?
Michael