What makes the Bloom around instruments . . .

Agree with Baranyi.
Also, are we all talking about the same thing? We have 'air', and 'bloom' Now bloom seems to be a better air, but it could be a term used to describe 'warmth' associated with tubes.
So I wanted to clarfy that I assume we are talking about the bloom which is a better form of air. And NOT bloom as a form of warmth.

I agree with Elizabeth, I think you've got it right.
I have more bloom using a really really good passive now, and with better hot rodded power filtration units. I think a lesser preamp can not get the bloom resolved also...

So bloom is being described more or less a type of glowing/radiating, of the air/sound around each instrument, not a tonal color variation?

I have been listening to the exact same Spectron you have for well over a year now and can tell you,as a former tube amp owner (BAT and Cary) that regardless of what reviews might say,or shall we say,reputation beckons,this amp has little to share with valve topography.Yes,topography,as in the multi dimensional relief of any surface,in this case the music as read by this amp as opposed to a tube amp.I understand what you mean by tube bloom and you will hear this on the Spectron,or something close to it,when a specific recording exhibits a magical,spatial quality and you are hearing the room the music was recorded in as the music hangs in the air.It really is one of those very special tinkerbell moments when you hear that and that is why this amp is so bleeping cool! Providing all the other stars are aligned in your system the Spectron is the mirror image of the recording and it's brilliance and downfalls.Yet it does one thing very,very well-it makes everything sound good and the great,well magnificent.
You know i miss that tube glow like a warm cat on your lap and i may return to it for the occasional fling, but i believe the Spectron will be my Pegasus.
May i suggest you investigate the Remote Sense cable option the Spectron affords.This will take you further down the path of better sound and instrument isolation and added topography.This is a must hear.

I also think Chuck's speaker cables and such are more resolving than your anti cables. So maybe more details/resolution are heard in Chuck's system.

Hi Joe. This is certainly possible, though I don't know that it effects the likelihood of either scenario I described in my last post.

I suppose there is an argument to be made that Chuck's system, if it is more resolving, might reveal characteristics of the XA30.5 that are inaudible in my system. But I have a hard time believing that any additional resolution would reveal "bloom" as a consistent characteristic of the amp. I could be wrong about that. I also could be wrong about what Chuck has in mind when he uses the term "bloom." It seems there is some difference of opinion about what that term means.

My description of the XA30.5 is based on hearing it in two different systems (mine and a friend's). I have also heard other Pass amps from the XA.5 series in other settings, including some systems that are very revealing by any standard. It is on the basis of those experiences that I have characterized the XA30.5 as "neutral," rather than having "bloom." Of course, other people familiar with the XA30.5 may have different experiences and different opinions, which I am certainly open to hearing.

Bryon