Technically, what causes "laid-back" or "up-front"

I think you have to ask the question laid back or forward compared to what. There are all kinds of reasons for certain sounds. Some are as much perceived as real(such as the case in Vandersteen's which measure essentially flat) Other speakers have a suckout usually in the crossover region that makes them sound dark or laid back. Some forward sounding speakers jump at you because the manufacturer has built in a upper mid to treble boost. Then there is the phase issue where drivers are actually out of phase to one another(Sonas Faber comes to mind here) Manufactures like to play games with speakers(as well as price) to offer something "Different". I wish I knew what a genuine "Neutral" speaker sounds like. Of course then---neutral to what! With the hundreds of speakers on the market and everyone claiming theirs to be the most accurate---somebody is wrong. Look at this sight and how many recommendations there are for speakers that someone feels is neutral. Well, we all can't be right. That's why we ALL buy something that WE like. What's neutral got to do with it? What's neutral to one is laid back to another and the sames goes for that forward perspective.

Laid back = a shallow broad dip in the midrange frequency response; whereas "up front" would indicate a broad peak in the midrange response. Even a mild rise or dip would be audible and manifested in the way you noted. Room reflections and cancellations can cause or exacerbate such effects too.

I believe the sonic holography thing had to do with electronically cancelling the interchannel crosstalk, which results in a very wide soundstage. If done perfectly, it would mimic a headphone's binaural presentation. Polk had some success with this as well. Neither method yielded perfect results but the effects were easy to hear.

You can also cancel interchannel crosstalk acoustically, by placing the front speakers about 3 feet apart and then putting a large acoustic barrier between them ( a mattress or door turned on its side would work) extending from the midpoint between the speakers to the listening position. You could position your head 6" back from the end of the acoustic barrier and listen. I heard such a demonstration of this effect and though the speakers are only 3 feet apart, they sound like they're more like 30 feet apart. Ralph Glasgal at ambiophonics.org has a lot of good info on this subject. Here's another link that gives further explanations and tells how to use a computer with a Soundblaster Live card to create a concert hall experience.
http://audio.muddle.de/

Enjoy!

I don't think placement had anything significant to do with the difference between these Avantgardes and the Avalons that were almost in the same spot. I really feel that there is something about the speakers that causes this.

I have often wondered the same thing as pbb - just HOW is it that some speakers have a considerable difference in image height? It's easy to understand the psychoacoustics of typical left-to-right imaging: if the sound comes soley from the left channel, it shows up over there, and similarly from the right. If the sound comes from both, it's easy to understand how the image would originate in between them. But how about image depth? Or height? These are clearly palpable things, and it's hard to mistake them. And how is it that on most high-end systems, the image clearly forms outside the angle of the speakers?

Something that the reproduction system is doing is fooling our brains into thinking that the image is *SO*, but what?

Related question: anyone know how the old Carver Sonic Hologram Generator worked? I'm sure it's related. For those that don't remember, this processor was inserted into the signal path and it basically added a small amount of cancellation to each channel and dramatically opened the soundstage. I used this up until my most recent upgrade, at which time the pure 2-channel electronics and speakers became capable of doing the same thing unassisted, but it produced a pretty good facsimile of a high-end image for distinctly mid-fi investment for over 18 years in my living room.

In human males, my guess would be age. In speakers, I would say the basic design, basically. Placement is a very good second guess. So one can try altering the placement and room acoustics, but it may not change the fundamental characteristics of the speakers' presentation. One thing that I notice between speakers is the great difference between how high the music appears to be off the ground, so to speak. Some speakers give me the distinct impression that I am close to the stage looking up, whereas others make me feel I am further out into the hall level with or looking down on the performance. Any thoughts on that? Good day.

Often times it will be the speaker placement that effects this. It can also be cables, amps and front end. So the easy answer is everything. I've changed my system with footwers and cables to adjust the presentation.