A few days since I asked the questions above, but I thought I'd report back some success.
I was rummaging around in my loft on Friday and discovered an old "mini" Hi-Fi in one corner. Thinking about my curiosity about surround sound, I unpacked it, and - sure enough - it had an auxiliary input. Whatever it was that I was looking for soon got forgotten and I shot downstairs, turned on the organ and had a play around with "audio output groups" and all that malarkey.
My, my, what a difference!
I could only load about half of the Zwolle instrument in four-channels at 16-bit resolution, so there are certainly improvements to be made when money allows. I also have to report some disparity between the 500-odd Watts of power on the front channels, versus only 15 Watts on the rear channels - the poor little Hi-Fi unit was turned up all the way to maximum, which in turn caused some crackling and distortion.
In spite of all that, it was sufficient to give me a taster of what playing a surround sound set is like: simply phoenomenal.
It wasn't until I turned the rear channels off that I realised quite how "flat" the two channel stereo sounds. With the two rear channels active and me positioned in the middle of the sound field, it really was like being in the building - a whole new level of realism which I've never experienced before. (I've never had surround sound for my TV or anything like that.)
I realise I've been satisfied with only a stereo layout in the past and it still sounds very good - that hasn't changed. The sad thing is that now, when I play any of the other "stereo only" sets I own, I'll always feel like something is missing. I expect that, in the case of the smaller instruments without long, lush reverb, it won't be so noticeable - but will I be able to play the other larger instruments again without wishing for surround sound? Probably not.
Ladies and gentlemen: I admit it - I'm an addict.
Now, the question is, how do I persuade my wife I need another couple of speakers of similar quality to the BX8's I have for the front channels....? Oh - and to the person who reported turning the sound level
down when converting to surround - I agree. I'll be doing the same when I get the chance to play with it.