Noise accumulates, but not that fast. If a preamp's background hiss is 20 dB SPL (masked by room noise in most environments), and, say, the equalizer's base noise is also 20 dB SPL, then the resultant base noise of both will be 23 dB SPL--also not audible in most rooms but barely noticeable as a difference even in a soundproof box.
But if one device produces a hiss at 50 dB SPL, then another device that produces a 20-dB hiss won't make any noticeable difference.
Quiescent hiss (produced in the absence of intentional signal) and noise aren't the same thing, of course.
Dynamic range is important. The base noise in my listening room is no better than about 35 dBA SPL. The background noise has a strong component of equipment rumble (I measure when the AC is running as a worst case), which A-weighting discounts. The same room is more like 55 dBC SPL. Given that the background noise is dominated by low frequencies, the minimum discernable musical signal--say, a solo violin in an orchestra context, muted, playing ppp--may be at 25 dB SPL, though we may prefer that it be louder. If the music has 60 dB dynamic range, the peaks--a cymbal crash or fortissimo trumpet--might be at 85 dB, which is loud but not overpoweringly so. LP records don't have this much, of course, but CD's might if recorded without any gain-riding or compression.
If simulating an orchestra-hall experience, the peaks might need to be more like 100 dB, so the quietest bits may be no less than 40 dB. A 40-dB (C-weighted) light tap on a bass drum might not be heard over my AC system, but the same 40 dB from a muted violin will be easy to hear. 40 dB of hiss might be fully masked.
But that cymbal crash has most assuredly been compressed at some point, either intentionally or as a byproduct of something in the recording chain. Even the recording studio room will probably damp the percussive peaks a bit. A really dynamic digital recording might preserve it better than in the past, but I doubt any recording has more than about 70 dB dynamic range. (I suspect most recordings are more like 20 to 30 if perceived as dynamic, and compressed pop music might be less than 10).
Even for those super-dynamic recordings, where we attempt to place peaks near live levels (say, 110 dB SPL or more), the quietest bits will have to be pulled out of the noise in a typical listening room, depending on where it is in the spectrum.
Predicted in-room response for loudspeakers is based on assumptions about early reflections. If a typical listening space has side walls of half the distance between the speaker and the listener, the reflecting angles will be 45 degrees off axis. If the side wall is the same distance as the speaker is from the listener, the angle will be 60 degrees. If the speakers are on the narrow end of a small room and the listener is at the other end of the room, the angle to the early reflections might be as little as 25 or 30 degrees. Thus, if we think that for most rooms, the early reflections will come from surfaces with a reflecting angle of 20-45 degrees, then the frequency response of the speaker at that off-axis angle can be measured, and will predict what the coloration of the early reflections will be. If the early reflections demonstrate a hump not seen on axis, the effect might be really annoying. There is an expectation that the early reflections will have less treble energy than the on-axis direct sound, because high frequencies beam somewhat even with the best waveguide designs. This drop in high-frequency energy off-axis predicts that the combined sound that might be seen on a spectrogram would tilt downward as the frequency increases. If it is smooth, people will hear it as a room effect and filter it out naturally. At least that's the idea. When you think about those angles, it's a fairly unusual room that would impose reflection angles (particularly laterally and from the floor) that would differ much from that "reflecting window" assumption. The more those reflections are damped, the less downward tilt in the in-room response, and the brighter the sound, but to my thinking not unrealistically so.
Movies with sound effects are a whole other consideration.
Rick "there is noise and there is noise" Denney