MEP,
What you say is true ... the circuitry aspects of the SE upgrade are a mystery to me too. I got the same explanation when I spoke to Kal before opting to have ARC do the SE upgrade to my Ref 150. Here's a couple of thoughts and observations.
First, Kal told me that the Ref 150 SE SQ is very close to the SQ of the GS 150. Kal mentioned that the Ref 150 SE upgrade incorporates many (most) of the circuitry enhancements made to the GS 150. If you have any interest in continuing your investigation, I suggest you ask Kal to confirm for yourself what he told me. The relevance to the Ref 75 SE is that the same genre of circuit changes made to the Ref 150 SE no doubt are being made to the Ref 75 SE. Here again, ask Kal.
Second, there one or two published reviews of the GS 150. The reviewer(s), as I recall, thought the GS 150 was a better sounding amp than the Ref 150 (not SE). You should be able to find the review(s) by checking the ARC web site or doing a web search. The take-a-way is that if the same circuit changes made to the GS 150 were indeed incorporated into the Ref 150 SE, and presumably, the Ref 75 SE, then you have additional corroborative date to confirm that the mystery changes are not snake oil.
Third. Take a look at Oregonpapa's Ref 75 SE comments on Audiogon. He just became engaged with his Ref 75 SE and plans to elope.

Another set of data points to consider.
Next to Last. Take a look at Marc Mickelson's side bar comments in Roy Gregory's September 19th review of the Ref 150 SE. I think I quoted MM's comments above. I think RG would like the Ref 75 SE better than the Ref 150 SE because the "75" has beautiful front analogue meters. SIC
Last point. Almost forget. ARC did the SE upgrade to my Ref 150. I think the amp sounds a lot better.
I am sure that whatever you decide will be a win-win. Good luck.
Bruce