It may well be that the original CLS 1 had a lower impedance at high frequencies. Usually how that behaves with any tube amp is that the amp will be rolled in the highs.
Our amps did not roll off with the CLS 1, the same as with other tube amps, but when you put those same amps on the later CLS models, they all rolled off.
So empirically it is logical to assume that the original model had a higher impedance. Based on this experience, I am inclined to think that ML's history of their products is not accurately portrayed, but if it is, then they had something quite unusual- perhaps an increase in efficiency that compensated for the lower impedance.
At any rate: the CLS 1 was an easy load for tubes where none of the succeeding models were.