Where do I position 4 speakers for 3D stereo sound

"However, I have found that the effect is all up front."

This could be how it was recorded. High-fidelity (faithful to the source) is one thing, special effects are another. You're basically doing surround sound with music, some discs have far more material on them for this endeavor than others.

"Which processor would be best for creating spacial ambiance through only the rear speakers using today's technology?"

Depends on the recording. There are Dolby Surround Music only recordings. However, you'll probably get the most mileage with one of the generic matrixing type devices to turn two into four--I can't remember who did what off hand. It could add some neat effects, but I'm not sure you're necessarily going to step into a concert hall with it. Ambisonics requires 4 or 5 microphones to do a proper recording w/ ss music.

You will have a rough time with the people here on the 'Gon with almost anything but two channel, analogue/vinyl tweaked out systems, being fed reconstituted power through special ac cords. So there I said it, again. Do I fell any better? No, not really. Briefly, just looked at the current issue of the British magazine "Hi-Fi +" on the newsstand today and, lo and behold, a new multi-channel decoder or enhancer or whatever ( I didn't actually read the article and was too cheap to buy the mag, preferring to spend my limited money on records instead) made by Dynavector. Interestingly, the ambiance channels seem to be on the sides of the room, somewhat next to the listening position, but, it seems, pointing forward, towards the main speakers. So, you see, your notion that, somehow, the sound would have more depth with four rather than two speakers, positioned fore and aft, actually makes so much sense that one of the most respected cartridge manufacturers has decided to come out with this type of product. I don't know what to tell you as to what may be available between what I have (and may put back in the system soon enough) which is no longer made and going the multi-channel SACD route, which may very well be premature. Since I have never been into 5.1 home theatre, the Dolby decoder, whatever it is thing, is a total unknown to me. Yes, I have lived through some kind of audiophile eclipse. So now, maybe, you can look up what Dynavector proposes, it might be the ticket for you, and don't let the buzzards get you down, it's your system to set up the way you enjoy it. The same nay Sayers would be saying nay about stereo if we set the clock back to mono days.

Just took a look at the Dynavector site and, indeed , the additional channels face forward towards the main speakers. The system is called Super-Stereo. I don't know much more about it, but look it up. Seems you need propriatery speakers though. Cheers.

Thanks for your suggestion, Pbb. I think you may have hit the mother load. I've spent hours today looking at the Dynavector website and was very pleased with their presentation. Hopefully I'll be able to use the Lascalas for the rear speakers. I'll check that out tomorrow.

The best place you can position 4 speakers for "3d-Stereo sound is stack one pair on top of the other pair. Try the best you can to match the output levels. Remember, stereo is a 2 channel format. It never was meant to have more than 2 channels, nor any surround sound additions. The music is not recorded that way, and anything you get back there is not correct.
Additionally, every conceivable method of 2 channel surround sound has been done, and re-done, a hundred times. I guess you weren't around in the 70's when this was being tried the first time around, and led to the awful Quadraphonic being introduced, only to die a couple years later. Hafler circuits, ADC digital delay units, "ambience" processors. They've all been done to death.
If you want "Stereo" than you want 2 channel. If you want surround sound, then move over the the HT world where "more is better, and even more is even better". Soon they will have 48.1 systems with 48 amp channels and 48 speakers, with one for the center, of course. For that "movie theater" sound. Yecchhh.