Component with most effect on soundstage/imaging?

Well, this is not a direct answer to the question..but the room and it's noise level is my choice.

Room size shape and acoustic properties, followed by the noise level both inside and outside of the home..have major impact on what you hear, esp. low level detail and decay that give much of the sence of space/image.

i think your findings coincide with mine...the amp and speakers are the most important components followed closely by the preamp. the source is a distant fourth and cables have never made a huge difference for me with respect to imaging. tweaks/isolation have made improvements in some cases.

You might want to find a technical book that describes how the soundstage illusion is actually created. Its something I should do. I don't know if the Audio Cyclopedia actually covers it some. I rememeber way back when, and I believe it was ambisonics, that built a retractable wall down the middle of your listening room running right up to the listeners face. This insured that sounds from the right speaker weren't getting to your left ear and sounds from the left speaker weren't getting to your right ear, I believe this nasty event is called crosstalk. Its an inaccuracy that nearly everyone lives with. Bob Carvers "sonic holography" circuit created out-of-phase signals from each speaker in an attempt to minimize the crosstalk, which would restore the soundstage. I don't know much about its workings, but it probably did a respectable job and it was a circuit that seemed to truly be restoring some accuracy to the source, as opposed to these generic "harmonic restoration" devices and Sound Retrieval Systems (SRS) which are different altogether. I believe they add an artifical sense of presence. And way back when Polk had a speaker that I think tried to correct this by having extra drivers on the front baffle angled in peculiar ways. But the crosstalk in most audio listening environments is one thing that gets little attention. This is one advantage to headphones: everything from the right channel goes only to the right ear and everything from the left goes only to the left ear. Strangely enough, headwize, or one of these headphone amplifiers incorporates a chip that tries to emulate the crosstalk found in most 2ch setups?!?! I guess they're trying to create an image one is accustomed to from loudspeakers, or in the headphone world maybe its designed to at least get the singers out of the middle of your head, even though the singers being there is accurate. However, I don't know if anything is ever done upstream in the recording phase to accomadate for this phenomona. But if you want to build that fifth wall to your room running between the speakers up to you that would help the soundstage. Good acoustic treatment can help clean up the soundstage some too. In the end I think electronics all the way upstream including speaker placement in the room are all important. I don't know if I could prioritize.

Eric since you're downsizing this probably isn't of particular interest; however for the others...
One cable "system" that significantly enhances staging is the Synergistic Research active shielding Master Control Center. This is the DC power supply that applies a static DC voltage onto the isolated shields of their interconnect cables. The staging ehnacement is its strongest feature, followed by improvements in bass, microdynamics, & blacker background. They even offer active shielded AC cords now, but I haven't yet had the pleasure. I'll be having mine upgraded when that service is available later on this year.

I agree with those who feel that the room itself bears a great responsibility for soundstaging and imaging. I feel that the most important aspect in this regard is where in the room you place your speakers. I have experienced amazing changes in depth perception by a small, few inches, speaker moves into the room. I also have had great luck with close to corner placements as far as the speaker disappearing act is concerned. Also I have experienced greater imaging and soundstage by placing my speakers on the long wall of a room. This did not make any sense to me when it was recommended, but it was an obvious improvement.
Good Luck.