I owned the Marantz PM 8004 for over a year. I just sold it; hopefully, the buyer will like it as much as I did! I replaced the PM 8004 with a Primaluna Prologue One tube integrated, which has added bloom and dimensionality to music that none of the class A/B SS amps I've tried could muster -- but that's another discussion!
Back to the Marantz PM 8004, I think all of the comments above have been spot on. The lower noise floor really does help contribute to more realistic sound in the way instruments just pop out of a black background; one can easily differentiate and follow individual instruments during songs, nothing ever sounds disorganized as a result. None of the other integrated amps I've had (nad c326, rega brio-r, peachtree decco, arcam 18, yba wa202) could compare in this regard. This quality set the pm 8004 apart and I usually noticed its absence, as well as its special clarity in the midrange and smoothness up top, when trying other amps in my system. In my mind, these tonal qualities allowed it to really excel playing vocal, jazz and classical music. On the other hand, if you value a bright, lively, and upbeat sort of sound, the marantz would not be my first pick. Finally, as others have also mentioned, the build quality, from the case, to quality of components inside, to the built in phono stage, to even the speaker inputs, is exceptional through and through. Based on the quality of the pm 8004, the Marantz reference range must be something else.