It's because there is the full 6dB baffle step compensation with bipolar. Add room gain to that and some things can get a bit too bass heavy. That's why it only worked with the little 5.25" drivers. My 6.5" bipole cube was a total no go with anything like Portishead and similar. Even well away from walls it was overblown. Lovely though on things you wish had more bass
For me it was an attempt to get more sensitivity by adding another driver (3dB extra). If you wireit in parallel with the other identical driver then that's another 3dB (6dB) total. Now I plan on using a more sensitive, slightly larger driver then that is not so much of an issue.
The options for me now are single up firing 6.5" mid bass run full range (no filter) or a down firing 6.5" bass with a low pass passive line level xover (1st order) at approx 250 Hz. That will need a dedicated mid driver from 250 hz up. It will of course need a high pass passive line level filter. The one I am thinking of has a smooth roll off so I will try and get away with not low passing it if possible. tweeter arrangement will stay the same.
If I xover at 250 Hz then that keeps the filters out of the range which our ears are most sensitive and the mid and bass drivers will still be within a quarter wavelength away from each other at xover even though they are on opposite ends of a 30cm sphere.
Just need to dust the mothballs off my wallet (again)