Just try it, perhaps in a store that has both receivers and preamps. You'll see. Yes, there is noise floor related difference but only relevant to me at 70db spl or higher. More important, is how the same A21 amp now controls the speakers (sounds of cello recordings, dynamic bursts in sound, for example) & that is perhaps more related to power reserve than noise floor.
The thing is, before I became more firm on the objective side I have had spent hours and hours trying.., but not in any stores because it would not be possible to do it "right" (by my standard). Been that, done that, just not possible, let alone the sales rep's typical blablabla...when trying to do somethng serious.
The funny thing too is, I did use my A21, as well as my 4BSST, and a pair of vintage Marantz separates pair that was once said to have that "golden sound", = warm I supposed. Also used other amps including my one and only NAD integrated, and several external DACs. No I don't have any Martin Logan speakers, but I did use a few different speakers including my 1028 Be, R900. LS50, among others. Never heard any easily audible difference, let alone night and day, again wish I was there to hear the difference you were hearing. Also funny (not literally) I was able to hear the difference between the speakers easily, I am positive I could have no trouble scoring well over 50/50 even if done blind but obviously I couldn't prove without help by others. I also thought I could hear differences between some of my DACs, go figure!!
And by the way, I had others presence too in some of the tests, including a near DBT to compare the 4BSST with the A21. The guy who scored barely better than 50/50 bought my Bryston, but he also wanted my A21, that I won't sell until I am ready for the AHB2. I do agree with the power reserve thing, if your night and day difference reference to that then it would be more understandable to me. Sometimes people would claim such difference even at low spl, that, to me would not have been possible if the tests were done truly apple to apple, and blinded.
This truly is a very interesting thing, and I am one who would not mind paying $100 for entrance to a DBT comparing amps, even though I am very confident of the outcome. As long as such a DBT meet my expectation in terms of protocol and the DUT list.