Interesting thing to try with a Dipole (like Magnepan etc) and a sub 

Hi Joppe. This is similar to the method I have used for my dml panels, for at least 10years. The difference is , I have my Subs up against the wall behind the panels. The distance difference from the front cone to the panel is about 30cm. I then time align them using dsp . The subs do not have rear wall reflections causing problems because they are up against the wall. Time aligning not only cancels the rear panel wave at source but also strengthens the front wave. I usually run my panels down to about 100hz, and the subs up to about 300hz, depending on which panels I am using. Doing this gets rid of the room and panel, suck outs which tend to appear in the 100hz to 400hz regions, depending on the panel used. I usually have the low frequency drivers ,probably at least 3db louder than the panels , this sounds more natural to my ears. Depending on the panel low end performance, the sub and the panel can both run together from 40hz to , in my case 300hz. I have not tried having the panels any further forward, mainly because a lack of space. I hope this is of use to you. Steve.

i always thought that ripoles are very good at this. You should be able to put a ripole on top of a regular sub and get a moderate but broadband cancellation. One series of woofers that I think could excel at this is the Peerless domes marked GBS, for example GBS250. They're discounted and very compact. If you could put a woofer like this in front of a regular sub, you should get a nice bandwidth. The scary part is that you always need to bump up the bass when going cardioid. It's the same as with dipoles, the lows are weak and lifeless without at least 3db of boost.

You should definitely go one and make a full range cardiod speaker, using a boxed woofer playing against the back wave of a your planar dipole sub dual membrane well into the mids, crossed into a ribbon tweeter in a controlled waveguide playing cardioid highs! Only the sub frequencies from the ported box would remain omni. Making it the first speaker with perfect dispersion down to 50hz! That would resolve the problem of the rear wall reflection!

Maybe I am wrong, but I would take two open baffle woofers and connect them in a 90 degree angle (magnets pointing to each other). Pointed forward it should make a cardioid source ? Well, a kind of .... I think.

Is this is a time to try MBL-like omni woofer but with your cylindrical planar membrane rather than MBLs dynamic driver and metal blades? You could manage a tension with increasing/decreasing a distance between that "drum" caps. The only a critical thing is a circular tension