Experiment, if you must, with care & VERY low volume.
The Spectral is a wide-bandwidth design and will thereby accept input signals in the MHz region. Spurious signals entering the system in the very high hi's could send the amp into oscillation.
That PARTICULAR mit cable allegedly rejects such signals, so is recommended. It cuold have been another cable producer, maybe that's where the friendship comes in...
For the record, I've tried Spectral amps w/out a pre and with another pre (through cdp). While I detected no problems -- visible (i.e. drive units excursions) or audible -- I was in a relatively "clean" zone... the system picked up noise from a cell phone though, so nothing is "invulnerable". And I was playing at "whisper" volumes just to be safe!