while i didn't plug the numbers into Unibox, the graph looks as it should for a ported enclosure. I think the "dip" you are referring to is the one of the graph labeled "SPL of the Driver"...you want to look at SPLtot
I think the only thing you didn't do is hit "Update graph".As Richard mentioned, the driver's dip is just the damping of its motion/output as the port takes over. The SPLTotal is the actual FR.
Your 260L box tuned to 20Hz looks pretty good, won't let me attach right now.