Jan, I think you are onto something very important here with your speaker and channel set-up. I can confirm what you noted about routing the "diffuse" channels to separate speakers from the "front" or direct channels: "adding these channels separately not mixing them as intended with the front channel improves the fidelity of these front channels. Like adding rear channel to front channel which enhances the front channels but now with a even further increased realism."
I have said it before and I will say it again: additional speakers can be added in a variety of ways to increase the realism and depth of any division, collection of ranks, or even individual ranks in an organ. In general, my experience proves what you said is true. For a sample set with surround channels, the effect is much more realistic if the different microphone positions--e.g., "front" and "rear" are not mixed through the same speakers but fed exclusively to different speaker banks. This is not possible to do with every sample set, of course. Even some so-called "surround sound" sample sets do not permit this. Augustine, for instance, designs his surround sample sets in such a way that a mixing desk appears on the console page of Hauptwerk allowing the user to mix a variety of microphone perspectives through only one stereo output. I have not seen a sample set of his that offers "front ranks," "diffuse ranks," and "rear ranks" the way that Sonus Paridisi implements these. You have to have those virtual ranks available on the audio and rank routing screen in Hauptwerk to be able to route them to separate speakers. Otherwise, you are stuck with just one stereo output. (In defense of Augustine, his recording techniques are so superior that I have not missed the absence of the virtual ranks in his sets.)
As I have expanded my sound system over the years--ongoing even now--what I consistently find is that I increase the realism every time I add additional speaker banks to a division. The basic premise of my system is to build up each division with preamps, amplifiers, outboard reverbs, and speakers just as one would do with wind chests, reservoirs, and pipe ranks in a real organ. When I have played and heard other Hauptwerk installations than my own, the biggest mistake I have repeatedly encountered is the premise that every organ should be recreated "as a whole." In other words, assuming that most organs are installed in one location in a church, the idea is to have all of the divisions--Great, Swell, Choir, Solo, and so on--play through a bunch of speakers placed at the front of the room. Then if there are surround channels, send the diffuse or distant channels to the speakers along the sides of the room, and send the rear channels to speakers in the rear of the room. I suppose if your music room is perfectly rectangular, this seems like the logical choice. However, my experience tells me that it is actually not the best choice, especially if you are lucky enough to have alcoves or whole other rooms coming off of the main space. Let me explain.
In my house, the entire main floor is given over to Hauptwerk. The main floor has a circular floor plan, which is ideal. The console sits on an inside corner of the living room. Three large speakers tower above the console. These typically channel the Great Division of an organ using the "front ranks" of a surround sample set. On the outside wall of the living room about 15 feet behind the organist are mounted 2 Canton bookshelf sized speakers. These reproduce the same front ranks as the 3 large speakers at the console except that those ranks are passed through an outboard reverb unit. This adds a vertical dimension to the sound and allows parameters such as reverberation and delay to be easily adjusted at will. The rear ranks of the Great Division are not sent to these speakers. Instead, they are routed to a different amplifier which feeds another pair of very large speakers mounted on the wall in an adjacent family room. The sound from that room emerges from one doorway into the entry hall of the house and thence into the living room, and from another doorway into the kitchen, then into the dining room, an finally into the living room from the other side. This approach creates a stunningly real surround ambiance. In fact, I typically route ALL the rear ranks--Great, Swell, Choir, etc.--through those large family room speakers. Then I build up each of the other divisions the same was as I did the Great.
For example, along the back or outside wall of the dining room are two discrete banks of speakers for the Swell Division. The first rank carries the "front ranks" of the Swell. The second bank carries the same front ranks but passed through another outboard reverb unit. Why do this rather than send the "rear ranks" or the "diffuse ranks" of the Swell to this second speaker bank? The short answer is because it sounds better in my system to do it the other way. And it gives me the flexibility to tailor the reverberation, color, and delay to my liking. (Since I am using HW4, I do not have the added features of the HW5 mixer to use for this.) There is already a good deal of ambiance and depth coming from all the rear ranks routed to the family room speakers. This allows me to focus on each division individually to further tailor its acoustic. On the far side of the living room are two more banks of speakers used in a similar fashion to set up the Choir or Positiv Division. Of course, a Solo Division could be routed there just as easily, depending on the particular instrument at hand.
The end result of this layout is that listeners hear each division of an organ as spatially discrete, while still blending very well with the others. To me it does not matter if the real instrument is not laid out this way in the church. Hauptwerk actually gives us the ability to reconfigure and revoice almost any organ--so why not make it sound better at home than it does in the church? This is not just a possibility. I have actually done it with one instrument after another. Or at least I can say this: to make an organ sound as good as possible in your home does not necessarily mean trying to duplicate how it sounds in church. Hauptwerk permits an incredible amount of experimentation and creativity in developing your soundscape. One thing I have discovered is that it is far more satisfying to a listener in my house to enjoy sound coming from spatially discrete divisions all over the house than it is to hear everything emerge from one location as at church.
Building a soundscape out division by division, using multiple speaker banks, and taking full advantage of an asymmetrical, multi-room environment are some of the key principles I have discovered in over 10 years of experimentation and system enhancement. I hope this information might prove helpful to some of you. But it does come with a caveat. Once you start down the yellow brick road of experimenting with sound layouts there really is no end to it! Have fun.