I'm not sure it's true that the M3 "has no crossover". There's certainly already at least a high-pass filter on the tweeter and the product page lists a 2.2k Hz crossover point for the M3v3. I don't think that the woofer - even if it is running full-range and rolling off mechanically above that point - is going to make much of a difference in your targeted area.
At any rate, if it was just a simple matter of changing the crossover to achieve better performance out of the system, why wouldn't Ian have done that in the first place?
To do this right, you would have to start from scratch, measure the drivers using appropriate methodology and equipment, and re-design the entire system. Might be an interesting exercise for an experienced speaker designer, but unlikely to yield a "better" system, IMO.
