I believe the words you are looking for are authenticity and believability. I beleive it is preferable to add something than to lose part of the original.

You might find it's honesty and directness somewhat in your face in association with the Bryston/PMC combination. You will most likely prefer something that takes the edge off the recording just a tiny amount, ever so slightly softening dynamics and drive. A passive should indeed fit the bill IME.

Should you get more natural sounding loudspeakers though, you might well find your preamp preferences change - such are the vaguaries of our delightful hobby.