Ronny,
The original prototype OB 5's and OB 7's had open sides, but still retained a back baffle, and top to the speaker box.
With the back and top on the original OB 5 and OB 7, there was a bump in the response that Danny had to address. He had to put in a notch filter (IIRC).
With the totally open versions, the notch filter wasn't needed, and the original network was causing a dip in the response, which caused a slight loss of body and fullness in the midrange.
With the new network for the open top version, Danny's restored the response so that the two versions measure the same.
That's why I highly recommended you do the new network. Your OB 5's have the network from the original prototype design, and therefore have a small dip in the response in the lower mids. Trust me, it's really worth it.

Cheers