In an ideal world, you would want to match your sub's characteristic to that of the main speakers. In this case, it isn't that they are planar that is so important but rather that they are dipolar. Unfortunately, there are few if any commercially available subs today (Audio Artistry used some in their systems, but I don't think these are available now). A custom builder might be willing to tackle such a challenge, but be forewarned that dipolar subs are quite large for their output capability... which might not matter to someone talking about using planars for a multichannel system.
Barring the dipolar route, which admittedly isn't going to be palatable for many people given the size, cost, and availability constraints, my next suggestion is to attempt to use a sub crossed below the room's fundamental mode, which is often in the 40-60 Hz range. If the Acoustats have acceptable output down to that frequency, a mismatch in radiation patterns from there down won't have detrimental effects on the sound (in fact, some view that as better than a full dipolar system) due to the room changing to pressure mode excitation.
If the Acoustats are not up to the task of providing acceptable bass down to that first mode, then you are left with a wide variety of subs on the market to attempt to match to the dipolar panels. I've personally had better luck with sealed designs when doing just that, but there is no technical reason why a ported design cannot also work well. What I found as the biggest problem is that, due to the power compression of typical panel speakers, the sub became proportionately unbalanced as the volume is raised higher and higher. I'd hate to suggest a power compressed sub to match though.
In the end, your ears (and his ears) will be the best guide if you can't go for the technically preferred solution. However, I will also strongly suggest examining room treatments and sub positioning, as this will play a vital
role in how a box sub integrates with dipolar speakers. Due to their radiation pattern, his Acoustats excite room modes far less than conventional speakers. A boxed sub lighting up those modes will make for terrible integration issues. Minimizing modal excitation in the lower frequencies will smooth the transition from one source type to another. Your job will be much easier if you properly address room acoustics, or very carefully place the sub at the least.