All i can say on this one is "Good luck" : ) Sean
Some kind of processing is needed or else what you will wind up with is sitting between two stereo pairs playing the same thing. To get any kind of ambiance, the rear speakers have to be fed a different signal than the front ones, or else they would have to be placed somewhere across the street (maybe across town) from your front speakers. Running the same signal, you will wind up with bass cancellation problems and, like Sean said, good luck. The simplest approach would be to use a Dynaco type set-up. The best way is an ambiance synthesizer. No one really makes these anymore for various reasons, none of which have to do with how effective the better ones were. Yamaha, Sony, JVC are the most memorable makes. I have owned a JVC XP 1010 for many years. It is not presently in my system, since I decided to be more of a two channel purist a while back. I am seriously thinking about getting it back into operation though, as it does provide an added degree of realism, especially to dry recordings. If you believe that one should not be limited to two channels, you should consider multi channel SACD. The problem there is that unlike synthesizing additional channels from stereo program material, you need a brand new collection of software. SACD recordings are still not very numerous. The manner in which they are going about multi channel is also quite perplexing. Just a quick explanation of the ITU standard for the speaker array they recommend and its placement gives one pause. You are right on one score though: less is more does not apply too well when it comes to channels in an audio system, purist be damned. Good day.
Without the right processing, 4 speakers cannot do 3d sound. Heck, without the right recording, 4 speakers cannot do 3d sound. You might get wall to wall sound or surround sound from all directions, but 3d sound??? Not really... 2 channel was created to effectively provide 3d sound. And it does it well if you have the right components.
Thanks your help. You have given me some good ideas. I too am a two channel purist, which is why I don't want any processors involved to achieve the effect that I want. I realize that good recordings provide a spacial effect with two speakers. However, I have found that the effect is all up front. I would like to feel that I'm involved with the music experience, not just witnessing it. Only back speakers can provide that. How to go about using back speakers is the problem. Processing two channel music through all 4 speakers is out of the question. Distortion of the original music will always be a problem, regardless of the quality of the processor. However, I never considered using a processor for only the rear speakers. I can now feel the gears grinding in my head.
Let me now bring up another series of questions. Which processor would be best for creating spacial ambiance through only the rear speakers using today's technology? Where should the rear speakers be positioned while using the processor? Is rear speaker distance to the sweetspot important?
"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.
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.
As a former owner (in another lifetime) of Pioneer 949 and Marantz 4400 receivers, ESS Satellite 4 speakers (bet few ever heard of THEM), a thorens TT with an AT quad cartridge, and a Teac reel-to-reel, I agree with Twl that they never quite got it "right". It was kinda fun hearing Keith Emerson's keyboard go spinning around the room, though.
I must respectfully disagree with your assertion that "only back speakers" can provide rear ambience. I find that, running 2 channel, I get a lot of rear information with recording that carry the ambience. I have rear speakers and a high quality HT processor/preamp, but I usually listen to music 2 channel only (front L/R speakers only).
Our brains will convince us of almost anything, if the clues are in the music (spacial clues are usually based on time and reverberation sounds).
Perhaps the recordings you typically listen to are lacking in this sort of ambient information. Try "Jazz at the Pawnshop" or "Missa In Tempore Pascale" - even Loreena McKennitt's "The Visit", turn off the lights and just listen - you'll hear a lot of sound from behind you (provided you room is reasonably setup).