Hello!
I added those top/bottom plates, and a steel wire crossing from lower back corner into upper corner. Construction is rigid now and plates are not bending anymore. Top plate bring some extra bass, so I had to move ob's a little bit forward to compensate it. Sound is amazing when listening for example good acoustical jazz music. I have been experimenting with LRC filter by changing resistor values. 5,6 ohms is a pretty good starting point but there are still recordings which would require higher values for most enjoyable listening experience. I do not have any acoustical treatment in the back of the OB's yet. It was mentioned somewhere that cork is a good material for it, but it's so expensive material here in Finland that I should find some other choices to try with.
