I agree with your observations, especially when it comes to the ability to create a holistic presentation. The soundstage is much wider and deeper with the new 333. When playing loud and/or when instruments come in hot (such as a trumpet) I could detect a slight harshness with the old Diablo. Now it is smoother without being veiled.
The first thing that struck me when I plugged in the 333 was the tremendous transient speed.
Whether it's due to giant transformers or a new architecture, I don't know. But I now experience the touches in the music as much more powerful, which contributes to an increased sense of presence and authenticity. I would liken it to what you get when you listen to music via a really good reel. It doesn't necessarily sound louder in terms of volume, but thanks to a much faster attack, it also becomes a physical experience.
If I was to give each one of them a character I'd say that the Diablo 300 is the loud and cocky youngster. Diablo 300 was a brutal piece that delivered high-octane music experiences. There was never a lack of power and when it came to playing bass, the Diablo 300 pointed with its whole hand.
The 333 is the adult version of the same creation. The basics remain, well embedded in the genes. But now the power and willingness to deliver in all situations has been combined with the kind of lived poise and confidence that comes with age.
The Diablo 333 is therefore able to deliver delicate, acoustic pieces of music both carefully and sympathetically. But when the volume goes up and the style of music gets tougher, the gloves come off and it's a completely different "devil" who takes his place in the arena.
As I had the DAC option in both my Diablo machines it is hart to say what each dac bring to the table on its own.
When I bought the Diablo 300 with the built in dac 7 ears ago, I already had an MSB Analog Dac with separate PowerBase, so I could compare them side-by-side. And to my ears, both played just as well digitally, i.e. speed and fan combined with organic weight. Then the MSB combo cost 50% more than the module.
In addition, as a consumer, I avoid spending money on signal and power cables. And the manufacturer does not have to produce a stylish but expensive chassis with a digital display to put the goodies in. Instead, they (hopefully) spend the money on good sound.
Fast forward to 2024 and the arrival of the 333. As the dac production was delayed the dealer offered me a Moon 680D streamer/dac (+10 000 USD price tag) as a loaner. After 3 months it was time to install the DAC3 module and do a quick comparison.
To my ears Moon 680D has a very pleasant delivery. There is nothing that sticks out or rubs, everything just flows.
It's very "likeable". But can be perceived as somewhat "kind".
In direct comparison, I feel that the DAC3 module offers a more open sound stage, even more space and air.
I mentioned earlier that I experienced Diablo 333 as transiently faster than its predecessor. Now there was even more dynamics and power in the notes, which contributes to the feeling of authenticity in what I listen to.
My set up consist of Sonus Faber Amati Futura, Diablo 333 w DAC, Nordost Vallhalla cables. My sources are Brinkmann Bardo, Antipodes K50 and a Studer B67 Reel2Reel.