In 'high performance loudspeakers' edited by Martin Colloms,there is a section devoted to panel resonance and measurement. An enclosure is arranged so that only the panel resonances are 'heard' while the driver output is trapped in a chamber.

The frequency response of the box is then plotted. Suprisingly, a 'reasonably well built' enclosures output was only -12dB lower than the driver output at certain frequencies(near 100hz from memory),and gradually reduced with increasing frequency.

To me,your loudspeakers simply cant be coloured in anyway unless you want it to be a musical instrument.

Linkwitz has gone without the box really,open baffle as have many.