A look at the spectral behaviour of a monopole and dipole ribbon explains some of the audible differences. The backwave trap is adding a significant amount of "damping" to the resonances. These measurements were taken at a distance of 2 meter from the drivers on a hard ceramic tile floor. Hence a significant amount of "floor bounce". The vertical scale is 4 dB per division to enhance the resolution.
The plot above is with a trapezoidal baffle. There are definately some similarities between the location of the resonance peaks as seen at 6 ms and the location of the harmonic distortion peaks as measured before. (800 Hz)
The plot below is for a monopole Carver ribbon. (The legend is wrong.) The resonances decay at a much faster rate. Definitely audible.
