The reason MIT is specified because the manufacturer wants to minimize uncertainty dealing with non-competent cable design frying the amplifier.
Spectral is a very high-bandwidth and high power design. Therefore, with a non-comforming speaker cable, the system may oscillate and fry the output stage.
With MIT, Spectral can make sure it presents a reasonable load.
If you find a purely resisitive speaker cable, and properly designed crossover in your speakers, you can use in lieu of MIT cable.
However, Spectral warranty will be voided if damage is caused by the non-conforming speaker cable.