Dan-
Do you think IMD becomes even more of a concern with very wide bandwidth drivers that cover multiple octaves (like your FRs for instance), more so even than flat BL?
IMD would increase, but it would be doppler IMD, which is not overly offensive to the ear (Doppler IMD is entirely even order). Over a wide bandwidth, flat BL is still extremely important because if it were not there, overlayed frequencies would be playing with different parameters.
Imagine a midrange is capable of 10mm of xmax (aka 70% of xmag) and has a Qts of 0.5. At 60hz, the midrange reaches 10mm of xmax. At 6khz, the midrange is only a 1mm xmax. Note that 70% of xmag will double your Qes (and thus your Qts). When the midrange is playing the 60hz and 6khz tones at the same time, it will be playing with two different parameter sets. For the 60hz tone, the driver will have an effective Qts of 1.0, but for the 6khz tone, Qts can still be close to rest (this is without considering thermal power compression, which has a similar effect on Qts/Qes). This is where distortion can come from wide range or full range drivers, so it is even more important to have ruler flat BL. That's my understanding of it anyways...
I guess I'm going off on a tangent, but I hope I helped with your question.
Neil
