Maybe the amp they used has marginal stability with cables without filters?
This from the dealer years ago:
"Whereas other Spectral Audio components (i.e. their preamplifiers and digital source components) may be used with any other brand of equipment, Spectral mandates that their non-Universal amplifiers be driven by a Spectral preamplifier to ensure reliable operation. Spectral Universal amplifiers however may be driven directly by any brand of preamp, D-A, or surround sound processor. In addition, Spectral amplifiers need to be used in conjunction with the proper Spectral-approved MIT interconnects and speaker cables. These usage guidelines are now a condition of Spectral's warranty on their amplifiers. There are several reasons for these guidelines:
Because of their wide bandwidth, Spectral amplifiers can amplify high frequency signals in the MHz range at massive power levels. Therefore radio frequency noise must be kept out of the amplifier. The MIT interconnects contain RF protection, and Spectral preamplifiers are stable at these high frequencies.
Unlike most all solid state preamps, Spectral preamplifiers are true discrete designs, are DC coupled, and can output the high current that the (non-Universal) Spectral power amps are designed to be driven by. Other brands of preamplifiers often use integrated circuits or other devices which will probably not be able to drive the amplifier properly.
As part of their stability design, Spectral amplifiers use an output network found in the proper MIT speaker cables. (Spectral has found that by placing this network close to the load, i.e. the speaker, they can achieve superior results.)
What will happen if you don't follow these guidelines? Well, it's a probability thing. You may not encounter any problems—or you may encounter premature failure of some components in the amplifier. Or (worst case) your amplifier may suffer severe RF problems, and you will have to pay to have all the output devices replaced (which is expensive). Therefore, we highly recommend that you follow Spectral's warranty and operational guidelines."
I don't know what a Spectral Universal amplifier is but when I had my DMA-180 I was told to use cables with filters (my dealer recommended Goldmund but he was both the Goldmund Europe distributor and the European importer of Spectral).
I wonder if, like the older Naim amps, the cable for use on Spectral amp needed a narrower than usual set of L, C and R characteristic for the amp to remain stable. I notice he had filter networks at the speaker end of his biwired cables, different for woofer and tweeter.