The output impedance of the Ref 75SE and Ref 80S is about 1 ohm in the low frequency space off the 8ohm tap, thereby resulting in a damping factor (DF) of 8 if the corresponding speaker input impedance is 8 ohms in the same low frequency space. By contrast, the Ref 150 had a DF of 14 off the 8 ohm tap.
My layperson understanding is that a "high'ish" DF helps to control the woofer and is a factor in yielding tight bass. I recall reading posts from some of our A'gon tech members that a DF over 20 does not incrementally add all that much by way of woofer control.
That said, I am sure that DF is not the only spec that drives quality bass. If Ralph Karsten (Atmasphere) catches this post, maybe he will explain (again) the various tech factors that contribute to quality bass. Such factor include, for example, in large the quality and input impedance of the speaker itself. For example, if a speaker has an input impedance of 16 ohms in the low frequency space, the DF would be approximately 12 if, for example, the amp's 16 ohm tap's output impedance is say 1.3 ohms.
In any case, enjoy your 75SE. It is a great amp!!