Personally I don't bother with that, TBH. My priorities gear-wise is to have gear which allows me to use:
a) crossover/integration of multiple subwoofers (and possibly external active crossover for main speakers)
b) room correction in the bass
c) easy access to basic eq and tone controls, for adjusting recordings to taste (some have too much bass and some have too little)
d) multichannel upmixing (that's a newer development for me personally, but I increasingly find it to be indispensable)
These things make a perceivable big difference for me, whereas SINAD numbers beyond a certain threshold don't. So I'm fine with a SINAD of 80 db, for example, as long as my gear does these other things which matters a whole lot to me when it comes sound reproduction and creating musical illusions in my brain. I'll take a higher SINAD if I can get it, of course, but I don't stress with it beyond a certain level. Priority for me is gear which does the other stuff well.
This is very interesting! Cool. Would you care to share more details about the setup, what the perceived differences were, etc? I don't find it strange that you could distinguish the AHB2 from the M180 tubes, nor that you couldn't distinguish the AHB2 and the Halcro beast (both super-magnicient AB amps). But it's intriguing that you could distinguish the AHB2 and the Ncore... I would like to know more, if you care to share. (my one concession to gut-based audiophoolery is that my gut is still somewhat skeptical to class d amps. Well-performing ab amps still feels like the safer choice - possibly with the exception of purifi).
I'll try.
System: RME ADI-2 PRO FS, my own MC/line preamp design, VPI classic SE TT with Mijajima Madake cartridge, VPI 3D arm. Modified ML Montis, with the bass path separated from the path to the panels. Raspberry Pi/Volumio, HSU ULS15 Mk ii. 4' long double 12 AWG cable from amps to speakers.
The Rogues have non-stock small signal tubes- selected to make the amps sound right to me, with the input of my spouse, who has a music degree and was trained in voice. An Otari MX50 with various 15IPS tapes was also used.
All of the power amps are mine, except for the Halcro units. The Halcro comparison took place at the home of a friend in Atlanta- he has a DM10/DM68 combo and a pair of stacked quad electrostatics. I brought the RME box and the AHB2s with me. He had an analog only (LP, Tape) system.
The Rogues sounded surprisingly close to the AHB2s- the differences were mainly at the very high end where they were a bit less "bright" and when the music became loud and complex they seemed somewhat congested and the image tended to flatten. The dynamics were also a bit more constrained, they have less of the "make you jump out of your seat in surprise" quality. The noise floor was obviously higher when digital sources were used, but it was not noticeable for tape or LP. The stock tubes sounded less like the AHB2.
The Ncore amps sounded, well, dull, in comparison- softer, less dynamic. It's hard to describe, but it was, apparently, obvious.
I had a friend who unbidden made exactly that observation when I was switching between the amps casually without any prior discussion on the matter. Perhaps the issue is some matching issue with the electrostatics. I tried measuring the frequency response, but I could not find anything unexpected- small signal -3db point at, if I remember correctly, about 50kHz. I can't measure large signal behavior.
I tried both analog and hires digital for sources. The conclusions were the same.
All Amps were initially tested with square waves to ensure stability into the speakers. No oscillation or unexpectedly long settling times were noted.