shifting soundstage>

that makes perfect sense; thanks.
does the "sweet spot" change with speaker placement?

Is it only on specific recordings and at specific moments in those recordings?  If so, then it's the recordings.  If it's somewhat random, then it could be equipment, but it could also be you.  If you listen for problems, then you're more likely to hear problems.

Ironically, it is more apparent on the most detailed recordings with the most specific and dimensional soundstage. On recordings with less focus it is not as noticeable, which makes sense. As mentioned, the placement shifts are subtle and noticeable only with listening concentration, so yes, it may be the result of critical listening.  In other words, yes, it could be me. 
The room is very large with an extremely high ceiling without acoustical treatment, which I suspect may play a role as well.

does the "sweet spot" change with speaker placement?

The sweet spot is more a plane than a spot. Technically its a circular section of a vertical plane, whatever the term is for that. There's a couple different things going on.

In terms of pure imaging its where you are equidistant to the speakers. You can toe the speakers in or out, or you can move your head forward or back, or up and down (that's why its a plane not a spot) and as long as you maintain equidistance there will be imaging. Move even slightly left or right and imaging suffers. At first it shifts towards the side you moved closer to but still sounds solid. Move further though and eventually you hear the sound coming from the speaker closest to you instead of magically floating in the room.

But its not only imaging changing. There's also frequency response from the speakers. There's usually a fairly narrow dispersion area within which the speaker sounds nice and smooth. Move a little left or right, up or down, this shifts the tone or balance. With some speakers hardly at all, with others quite a lot. This frequency response shift will of course have some influence on imaging, affecting sounds falling within the affected frequency range. This could cause certain instruments playing certain notes to seem to drift or move.

That's just the direct sound coming right off the speaker. A lot of what you hear includes sounds reflected off all the various surfaces in the room. The size, shape, composition and location of each of these contributes to the total sound. That is why they make dispersion panels, lots of odd shapes designed to break up and reflect sound uniformly with regard to frequency. That is why they make absorption panels designed to absorb certain frequencies.

A typical room has all kinds of stuff going on to the point it seems pretty random, and it sort of is. But listen real close you can start to notice things, and that could well be what is going on here. I still think its almost all recording, but all these other factors are in play as well.

Just to give you some idea what I'm talking about, my room was at one time pretty much empty of everything but the system and a chair. I had a friend over and left a couple albums out leaned against a wall as my pathetic attempt at decorating. He said wow awesome imaging but the left side not quite as solid as the right. WTF?!?!? Never heard that before. But sure enough he was right. How had I missed that?? He leaves and its bugging me until, wait a minute- the album! Put the album away. Rock solid. That one stupid square foot of flat surface was reflecting just enough sound to mess with imaging. So these things really do matter.

Now the way to tell what is what, if the shifting locations maintain their integrity as they move then they are legit and on the recording. Carlos Santana as his guitar moves hard left to hard right sounds crazy crystal clear every bit of the way. But if you hear something rock solid in one spot and then as they play it shifts and at the same time becomes a bit more diffuse and hard to localize, that most likely is the room.

pfrank, yes. Putting the speakers closer together will give you a slightly wider sweet spot as will moving the listening position farther away from the speakers. If a system does not have a sweet spot it is actually defective in some way. The more noticeable the sweet spot the better the system is. IMHE speakers that are more directional like horns and ESLs have a more distinct sweet spot. The problem then becomes that the high frequencies roll of dramatically as you move off center. Ideally the tonal balance should remain the same over the entire listening area. But, the image will only be perfect on the perpendicular line centered between the speakers. So if I want to check out the image and balance while someone is in my listening position I will stand just behind.