Peter, it's interesting to read that you found the difference between the XP-20 and the XP-30 to lie mainly in the voicing and not in a general difference in quality. I had them both here in parallel for a couple of weeks before I sold off the XP-20 and did quite a bit of comparison.
And here in my system I had the impression that the XP-30 shows not only a bit but actually significantly more detail and resolution on both the macro level (i.e. size and especially depth of soundstage, air between the instruments, three-dimensionality of instruments and singers, and realism of the presentation as such) as well as on the micro level (i.e. sheer amount of details to be heard). Also the bass was stronger, deeper and more powerful. I must admit that to me and in my system it seemed like a significant difference in quality and not only like a difference in the way of presentation.
On the other hand I agree that the XP-20 seems to have a slightly brighter voicing and sometimes even a bit more punch compared to the XP-30. To me it seemed to be that way because the XP-20 is quite a bit more compact in its presentation and it is what I would call more "transistor-like" compared to the somewhat "tube-like" quality of the XP-30. And this compactness combined with the slightly brighter presentation creates the impression of a somewhat speedier and punchier sound. Maybe not the worst way to go if you are into pop and rock or if your system is on the darker side of voicing.
Also, in my system the XP-20 puts quite a focus on singers in the middle of the stage while the XP-30 tends to accentuate more the stage as a whole and all that's going on there. In this the presentation of the XP-30 is definitely the more realistic one but the way the XP-20 does it is definitely fun.
All in all for me I prefer the way the XP-30 is doing things. I love the sheer amount of detail and the gigantic soundstage and also - mostly - prefer the greater musical flow of the XP-30 to the punchier way of the XP-20.
And I definitely believe that the Xs Pre does take all that quite a notch further up maybe even combining the best of both worlds on an even higher level. But the price of it is also definitely in nosebleed territory.