What makes the Bloom around instruments . . .

RX8 I have heard this sound. My father played the violin and would demonstrate the difference between a good instrument and a great one. The better instrument had a glow to the sound that was irrestible. The inferior instrument would just play the notes. Hope this helps. Bob

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.