I think most of the relevant points have been made here, but I will add a few thoughts. There was an interesting thread about voicing a few years back, which you can find here.
viewtopic.php?f=1&t=14111 I agree with the others here who said you have to do your voicing with the sound equipment that you will be using most when you play, whether it be speakers or headphones. NB: If you have several versions of the organ (dry/wet, front/back, etc) your voicing will not transfer from one version to the other! I learned my lesson early on with my first sample set, Ménéstérol, which had multiple versions including a third-party extended set. I had to redo the adjustments for each version. If you use speakers while voicing make sure you are sitting in the exact same position all the time, especially your head alignment, as this makes a big difference in perception. Oddly, sometimes a sound which is too loud can sound too soft, or vice versa because of our alignment.
One mistake I made which caused me to redo some voicing was having the subwoofer on while I used my headphones. Of course the result was skewed.
It is interesting to be able to compare the results between versions, not just dry/wet etc. which sound different anyway, but with similar-sounding versions, for example an original and a premium version. I often check this way to see if I like my altered results or if I have gone completely astray. Of course this is time-consuming…
I find my most successful alterations are in evening up the volume of pipes within a rank, especially pedal notes, and making individual ranks louder or softer. Happy voicing!