Well stated…
I almost always perceive upgrades that others report as revelatory as incremental.
If I live with the new for a while and then go back to the old and sense a loss of something I preferred in the new, then the new stays and out with the old.
Enjoy.
I experienced about the same as you when I went from the M700s to the M1200s. More detail, better bass, and improved soundstage. You are right, No going back!
There is a fullness to the sound at lower volumes, where the volume level does not need to be set high to experience sonic bliss. Someone else mentioned that music seems to just pop from the speakers with the M1200’s in their system.
Every time I start listening to my system, M1200’s running Magnepan 1.7i’s, I get goose pimples…
As I continue to closely listen to the M1200s with my rather eclectic music library, my love for these amps keeps growing. I will admit it got even better when I quit the back and forth with the M700s.
I’ve also been playing with my speaker placements with different music and am finding a small reduction on the toe in from my M700 position has widened my soundstage and caused the bass to spread out without losing any strength, if that makes sense.
Just keep loving these puppies!
At some point you’ll be able to hear the bass where it was recorded. e.g. if a double bass was centrestage in the recording, that’s where you’d hear it. Speaker placement will unleash capabilities you never thought your system had 
You’re not alone. I’m the same way.
In a couple of characteristically frank videos, John Darko has commented that the differences between various components is typically very subtle, and it’s the reviewer’s job to exaggerate them quantitatively so the viewers/readers can get a better understanding of the qualitative nature of those changes.
Don’t get me wrong - I’ve had night-and-day, immediately discernable improvements when changing components. Like going from an Emotiva amp to the M700s. The last change I had like that was when I changed from the DAC in a Node 2i to the one in my SGCD. But most of the time, like when I changed from the SGCD to a DSD Jr, the differences are subtle at best. As you say, I suspect I’ll only know the true extent of the improvements brought by the Junior if and when I ever feel like switching back to the SGCD for comparison.
Some great points there, and the Darko reference sounds super honest. I mean, when it’s a game of diminishing returns, as so very many acknowledge, it seems that the incremental improvement is just entirely more likely. Then again, removing the stupid ground loop in my home theater (no M1200s there) would be a complete revelation. 
My M1200s are becoming, and maybe have been, the weak link in my system for a year or two. I can’t imagine what would be a step-up that is reasonably priced. That is, low efficient speakers means I need the umph. Umph is $$!
Speaking of money, I imagine our music addiction is costing us considerably less than class A amps this winter with kilowatt hours being crazy expensive. I know I had the biggest Xcel bill (an energy provider for our non-US friends) I’ve had last month. I like the idea of staying class D for efficiency.
Differences in cables, for me, have been more subtle differences, though I admit I have not tried cables more expensive than my entire system, lol.
But the improvement, for me, that the M1200’s brought, was not subtle. I’m a tube guy that has been looking to minimize the number of tubes needed to be replaced every 1 to 2 years, and have been also testing to see what components work well with my relatively new Sonist speakers. If the M1200’s had existed when I purchased the S300, then upgraded to the M700, I might have started with the M1200. The improvements to female vocals that the M1200’s have over the M700 is, to me, not subtle, especially when the M1200’s are fed by a Conrad Johnson tube preamp. The natural timbre of violins, piano and saxophone, to me, is not subtle. While the M700’s sound good, the 2nd set of tubes in the system made all the difference in the world.
At the same time, you are spot on regarding reviewers exaggerating differences.
My memory has failed me. What preamp feeds your M1200’s and what are your speakers?
It’s conveniently in my profile. BHK pre → M1200 - > ICONOCLAST OFE (gen 1) → Harbeth 40.2 Anniversaries.
What did you previously have driving the Harbeths? Tube? S.S?
Different speakers (B&W 804d3) and different amp (MC462). I was either going 40.2 or Maggie w the M1200. I like the warmth and perhaps less finicky placement of the 40.2. Darren voiced the M1200s with Harbeths, and let me tell ya, I’ve been pleased for a long, long time already.
Yourself?!
I switch back and forth between the newer Sonist Audio Concerto 4 and the original 1990 Infinity Modulus, though have connected 2 SVS subs to go with either system instead of the Infinity Sub. Each speaker system has different strengths and weaknesses. The Sonist has incredible magic at times with tubes playing jazz & classical & female vocals, but then sounds better with solid state for rock and other cleaner music. The Infinity’s sound good with tubes for all kinds of music, but does not have anywhere near the magic of the Sonist speakers playing jazz.
I didn’t really feel the improved bass was the driving force for the M1200 upgrade. The M700 was pretty good pushing the bottom end. I was looking for the mid and high improvement with the tubes. And I got that for sure, and it seems to get better as they settle in.
BUT, as you say, the bass is a whole different animal now. Tight, thumping, fast, effortless. WOW!
I’ve upgraded amps for more power but were A/B technology and thought anything over maybe 250 watts was overkill. WRONG!
The M1200 is so much more than horsepower.
Thank you, Darren.