What makes the Bloom around instruments . . .

The Pass is class "A"

Without looking into the tech details of your post I would've answered 'Skills and spirits of musicians'.

For me, "Bloom" is just lower levels of unpleasant, low level distortion, which your ear has to try to ignore so that you enjoy the music. Virtually all systems inject a decent amount of this distortion, that classic dealer showroom "in your face" hifi quality has high levels of such.

The power supplies of Class A amps have a far easier job to do, so if all else is equal there will be less of this distortion, and more "Bloom".

Frank

I would say that "bloom" around instuments is there in the recording.. just most systems cannot allow it to pass from source to speaker. I would say the very finest small signal is where the 'bloom' resides, and it is too subtle for most amplifiers to be able to amplify properly. It gets turned into a general background haze, instead of being what it is.
Atually i think this is the one area where most of the usual electronics can keep it, and the amplifier is the weak link that just messes it up. IMO.

Elizabeth, I agree with your comments generally, except that I would say the amplifiers are not able to process the fine signal properly because of weaknesses, or problems, in the amplifier and its interaction with other components. Eliminate those weaknesses, and the amp will do its job fine.

And "general background haze" is just one of the audible indicators of, or ways of describing the unpleasant, low level distortion that seems to afflict most systems.

Frank