Hi Felix. I can see on a smaller instrument using the flutes for volume if you think it's necessary, but I still tend to find it an objectionable sound. And I was referring to pipe organs as well, not just digital organs.
My decision about the Subbass was primarily a St. George's thing, although I began to follow that method on other organs as I liked the results. I tended not to use it until both Swell and Great were registered past the mixtures, usually waiting until there was either the Great 8' trumpet or Swell 16' trumpet drawn. As I said, St. G's balanced beautifully that way, and the Subbass wasn't necessary in my opinion. I found it too heavy.
The congregation doesn't always follow the volume changes in the organ, but sometimes they do. Jerusalem is one of those examples where they do, and actually showed some sensitivity to the words. I found closing the box in the middle helped the big buildup to the high E.
Honestly, there aren't too many hymns where I actually touch the Swell box, this was just an example that happened to come up. For the final verse of "O Come, all ye Faithful", I use the box right at the end. I usually go to the swell for the first and second instances of "o come let us adore him" with the box only half open, then open, while I stack the Great and Pedal with everything under the sun, so that that crashing chord, (I believe it's and augmented E major chord) has a huge impact. I don't think you can use the box extensively - it just gets tiring - but thoughtfully used adds another dimension to hymns that can benefit.
I wasn't impressed with Diane Bish's version of that either. But then, I'm not overly impressed with Diane Bish at all. Yes, she's a fine organist, and I won't deny the talent, but something about how she does things just doesn't do it for me.
My preference to leave out the flutes comes partly from my training early on. I suppose I learned not to like it, if that makes sense. As I was always exposed to pure principal chorus, the addition of the flutes was jarring. I can see using an 8' flute to warm up a prinicipal for solo work, but in general chorus, it doesn't work for me.
Volume is one factor. But there is more reason than that. It's to do with word painting. I wouldn't doubt the possibility of the combination of Principal 8', Octave 4' and Waldflote 2', for example. If you tell me, one should not mix the Waldflote in, just because the other two are principal tone, that is hardly convincing. There is also a well-known registration, where a Flute 8' is mixed with an Octave 4' for artistic reasons. Again if you say that is objectionable, that may have limited the rich tonal possibilities of the organ.
About the Subbass, I don't fully catch what you said. Do you mean you would pedal with an 8' stop, but seldom with the Subbass 16? So you were saying unless you had a Mixture stop drawn on the Great or the Swell, you would not add the Subbass to the pedal. But earlier, you were saying that you would go for Principal 16', which is even stronger than the Subbass. However in hymn tune playing, I think the Protestant standard is commonly taught that the pedal is always used in congregational singing, though if you say you can bear with the Principal 16' tone for all the hymns, that's your organ. But I do know certain Roman Catholics have formerly remarked negatively on consistent pedalling throughout a hymn -- calling such efforts the results of "unpleasant sounds".
On those two English hymns you cited, Jerusalem and O Come, All Ye Faithful, I agree that the Swell is always used. The Prom Concert makes it such a familiar idea that there are dynamics changes throughout. O Come, All Ye Faithful is an example of common experience in every Anglican Cathedral on Christmas Eve. It is tritely done every time the Willcocks' arrangements are played. I guess he gave the dynamics hairpins in all the designated places in his music.
The thing is, I feel they have made a meal out of a molehill, so to speak, with the Jerusalem hymn. It is a satirical work about England. Some people say that should be made a national anthem. In all honesty, I think chances are close to zero, from a nationalistic point of view, if considering it be used to replace God Save the Queen. I am not saying that it should not be sung at all. It can be used as a hymn, but should not be the national anthem. Fine music, just on the words, I wonder if the British really understand what they are singing. I don't know -- maybe it was deliberate. I think the dynamics changes are exactly the meal out of the molehill for that one. I also think the Posaune 16' for the pedal for the ending postlude is rather unnecessary, too, although I once thought it could be interesting.
Something about Diane Bish doesn't work for you? Is she a difficult woman? I only know her show is on every Wednesday morning here, and know about her playing. Other than that, I know nothing about her. I am planning to purchase some of her CDs next year, as a matter of fact. She seems to have great interest in the organs of Germany.
In conclusion, are you in essence saying that one should not double the principal with a flute of the same pitch, or are you saying that flutes should not be mixed with principal stops in general? If it is the latter, is it bigotry or just your musical taste, or did your teacher not allow you to do that?