I see, devices which give axisymmetric and plane waves doesn't seem easy to find. A compressiondriver has like 105dB sensitivity while the 15" bass lies in the 96-99dB region. I'm thinking the extra sensitivity of the compressiondriver isn't needed since it'll have to be attenuated in the crossover anyway. Is it possible to improve the compressiondriver (more linear?) but sacrificing some sensitivity? But I guess building/improving compressiondrivers is out of the scope for a DIY:er