Agreed, there are a lot of variables, however I remember clearly that reference level on my H/K was set at master volume of -5 for 85dB. It's still at -5, however the trims are now way down (0-3-0 from 8-7-9) I don't have a tripod to mount with.....just use the "scoot down on couch real far" approach. Admittedly not a perfect scenario, however i'm as careful as possible to always maintain the same position.
I really don't know what caused the need for the change and have no desire to jump to conclusions. My SPL is new, purhased from R/S a couple months ago. I have noted the H-10 to briefly activate it's voltage regulation feature as I'm firing everything up, however it only boosts voltage for less than one second before everything evens out.
I don't want to add to the "snake oil" side of things when it comes to this type of equipment, so, next time I end up having to dig into the spaghetti back there, perhaps I'll try an A/B test as suggested.
Like I said at the start, I wasn't planning on seeing any difference due to this piece of equipment and added it for some very specific reasons. For now, I'll just continue enjoying!