I coming into this thread somewhat late, but it seems that this discussion is overlooking an obvious (at least to me) answer to the original question of whether the preamp or the amp contributes to soundstage and imaging. One poster, agarwalro, already mentioned that recording quality determines much of the soundstage and imaging we hear at home. Yes, without any doubt.
In my experience, more than the various electronic components in a home audio system, it is the speakers, and their interaction with the room they are in, that contribute most to the 3-dimensional illusion that we call soundstage & imaging. More specifically, it is the speakers' abitilty to handle large changes in SPL, or their ability to respond to a wide dynamic range in the recording. Speakers with larger dynamic range create better imaging and soundstage than speakers with smaller dynamic range. You cannot ignore amplifier power when discussing this because different speakers often require different levels of power before they deliver their best dynamic range in a given room.
tomd51 discussed how he swapped out 4 different receivers while using the same speakers. I find it interesting how he spent his money and time doing all that, and not surprisingly, found that he observed differences in the sound. Other people report similar findings when they swap out speakers, CD players, preamps, amps, RCA interconnects
, speaker wire
, or even speaker wire insulation
!! This is what mtrycraft is referring to as bias. What ever you work at most is, most likely, what you are going to observe as making a difference.
All this doesn't answer the original question. I'm inclined to believe that speakers, given sufficient power to satisfy their power requirements, provide the best way to achieve better soundstage and imaging. But then, I'm the kind of guy who likes to swap out speakers, not recievers or preamps.
