[William Sommerwerck]

I've sometimes been thinking about exactly what you say here,
i.e. a rear setup creating a natural ambiace, as it happens in the
real theatre/hall due to reflections. Kindof a minimal effect, just to
add what a spaciousness-wise 'flat' stereo recording don't have.

Yes, I've tried some crappy consumer gear attempting to do that,
of cause to no avail. I'll have a look at your suggested gear.

Uhh ohh, I'm most surely going to be lectured now :-D

Not from me.

You can start with a Hafler difference-signal setup while you're looking for
a synthesizer. (The Yamaha DSP-1 shows up all the time on eBay; just be
patient and wait for one with a remote control. If a Yamaha DSP-3000 or JVC
XP-A1000 or XP-A1010 shows up, grab it. JVC still has remote controls,
though they're down to three.)

I should point out that the most-significant ambience is the "lateral" sound
of the hall, not the rear reflections. All the synthesizers I mentioned
produce four channels of ambience, two of which are intended to come from
the sides.