What makes the Bloom around instruments . . .

I recently tried a Pass XA30.5 amp in place of my Spectron Musician III Mk 2.

In my particular system, the Spectron outclassed the Pass in every category except one: that magical Bloom surrounding each instrument and vocal entity.

I really liked that Bloom and I would like to understand how and why it's there because it is something very special and I'd really like to have it again in addition to everything the Spectron brings.

Thanks,
Chuck