Bipoles don't have baffle step loss. That combined with room gain puts lower frequencies with 2-3db extra.
A bipole does have a bipole dip. They need to be used well out into the room, so room gain is not usually significant except at really low frequencies where the box is already down in level.
A crossover can obviously bring it down.
A low pass filter on the rear driver can help avoid the bipole dip and still have baffle step compensation. With no HF you could put them closer to the wall where LF lift might be an issue.
So when you switch to dipole you have baffle step loss
When you wire as a dipole, you have 2 drivers essentially pretending to be a single driver on a board, bass cancels, the box does little. With the narrowness of the box and the likelyhood of the driver being lowish Q, you would need a helper bass driver to have any bass.
dave