What makes the Bloom around instruments . . .

i think bloom represents a balance between fundamental and harmonics.

using a violin or acoustic guitaras an example, bloom would exist when the wood body of the instruments was present as one observed the plucking of strings.

i don't think bloom is a treble phenomenon but rather what happens in the upper bass/lower midrange.

I don't think either the spectron or pass amps have the frequency response characteristsics i have described, i.e., i think that both amps have a transistory sound.

i have heard the spectron driving the analysis epsilon. the sound was somewaht thin.

i have owned a pass amp, but not the one discussed here.
i have heard the pass x series amps at ces.

i doubt any solid state amp can "produce" bloom.

i think classic tube products are more likely to be associated with the term "bloom", as i have described it than either current generation tube or solid state.

it could be that the pass amp is more linear in its frequency response than the spectron, which sound a bit peaky in the treble.

keep in mind my experienece listening at home is panel speakers, including quads (esl and 63s), magnepan (1.65 and 2.7) and magneplanr 1 bs.