Improving imaging

I also vote for speakers - but repositioning can also help. I now have mine towed in at about 30 degrees - mine are about 12' apart. I have KEF Reference 3's that I purchased because I liked the soundstage - they didn't have the lowest frequency response but they sound good and are fairly accurate. I have also listened to Meadowlark speakers and liked what I heard - they are also efficient and reasonable in cost.
I believe some tube amps also present a nice soundstage when working with a good speaker. I like the Cary single-ended (SE) amps but tubes do take some tweaking.
Shop with some CD's you like and trial speakers in your own environment if possible - quality dealers usually will work with you on this basis.
Good luck and good listening!

Yes, I have had imaging problems (mainly a center image that was no longer on center) that were/was remedied by simply reinstalling the cables a second time.

I have noticed when upgrading my modest system images became more distant.
Deeper soundstage, laid back sound. When you see these words used to describe many expensive components, it means small image. The further the stage the smaller the image, and you lose image height and layering (on most recordings).
That's why studio recordings sell and "recorded live" usually do not.
However a lot of audiophiles and reviewers seem to prefer that row "Z" listening position for some reason. Maybe it is more accurate.
I have some SACD's (very pleased overall). On one Santana recording, I think it may be the last track, the vocals are barely audible he is so far back. And Tony Bennet Unplugged-recorded space is verly realistic, but it is though I am in really bad seats! Sounds better in my car.

When I first got my speaker placement and room acoustics working right the sound stage was large and individual "voices" were apparent. I thought this was what I was looking for. Over the past two years I've played with a number of areas of my system, please search "winter lessons" that I posted a while back. I outline a number of "tweaks" that have worked for me.
To give a quick overview, assuming the basic system is adequate (you had imaging once) and your speakers are properly placed, I have found power cords, power conditioning, interconnects, footers and shelving can add up to equal at least a new amp. Today my imaging is simply fantastic. I do not need to "look" for the instrument it's simply there. The stage is wide and deep with complete definition through out. Each "voice has space around the sound, even in congested passages. The difference from when I first felt I had it and today is basically the difference from a mid-fi to hi-end system. There is a huge amount you can do to help bring the best out of the system you have. I recommend experimenting with your system, start where the above posts are starting, make sure your equipment is working well and is capable of what your looking for. Then the real work should start, try stuff, ask a local dealer if you can take things home (power cords, conditioners, footers) and experiment. Be sure the products you try are broken in or you'll have no idea what it's effect on your system was. Search "winter lessons", "shelving" and "Nueance". Redkiwi has written some excellent posts on shelving (above) and must be read. Good-luck and enjoy the music! J.D.

Thank you for the interesting & informative responses. Yes, while some things have remained the same (speakers & amp & interconnects) pretty much everything else has changed (but for the better I'd hoped).

Mainly, I've moved which obviously implies new/different room acoustics. While it is indeed a 'variable change', now it's pretty much a constant (and I'll just have to work around it).

You've confirmed my belief that overall it's the speakers being the most important component. However, these same speakers provided good imaging before so I'm not sure it's them (unless they themselves are/have degraded over the past 5 or so years). I've tweeked/optimized the speaker placement, toe-in, levels, etc. and the best results are speakers that are approx 9 ft apart, with about a 30deg toe-in which focuses them on a 'sweet spot' at ear level about 12' in front. Since imaging has otherwise gone from being present to virtually non-existant makes me think there something else is the 'weak link' in the chain.

I'm going to take a good hard look at my interconnects & cabling. They've just 'sat there', sans any maintenance at all so it's likely they could use a good cleaning or maybe even replacement.

Beyond that, it could very well be that while attempting to upgrade my pre-amp, sources, etc. I may have indeed disrupted my overall 'system synergy'. Now, since overall component synergy is more 'by the grace of the gods' than formulaic, how to rectify.