What makes the Bloom around instruments . . .

That definition of bloom by J. Gordon Holt correlates perfectly with Baranyi`s experience with his father`s violin comparisions.This presence of bloom is real and I`m aware of it when listening to live acoustic instruments be it a concert or friends practicing(piano,guitar,horns etc.)A good recording can capture some of this quaility and very good systems can reproduce it in your home. Based on the OP`s descrition aboveit seems the Pass amp has superior low level resolution/lower sound floor that`s allowing him to hear the natural bloom. The Spectron amp for what ever reason is`nt as resolved despite it`s other apparent advantages.

Hello,
Frogman that was an excellent description of what`s called bloom. It`s most certainly a function of harmonic overtone/fundamental tones and when captured is utterly beautiful. It makes music sound more realistic,thus less canned and sterile. It is a reflection of a component`s noise floor and ability to retrieve low level detail/resolution. How the Pass amp can do this well yet bury the organ in the musical track as the OP stated seems very strange, as preserving the bloom quaility is the more elusive trait.