Jax, Just for fun I put the Hewitt back on and I also played the Kissin's Carnagy Hall performance of Chopin Vol 1 which does not have Impromptus 4 but I assume its the same performance/recording session that produced Impromptus 4.
Without commenting on the performance itself, the sound differential was very interesting. The Kissin lower registers were in full bloom (big and warm) and the image was larger and more forward. The Hewlett was more laid back and had more of the highs dominating the image which did appear smaller/more recessed. The highs in the Kissin recording seemed less evident. Completely different recording styles. I think you will love the Kissin.
I guess I'll have to buy Vol 2 to get the Improptus (I assume that is where it is but I'll check). But, as to gain, the highs in the Hewlitt were very close to the same level as the bass in the Kissin. Choice of adjectives, but one might consider the Kissin recording as having more subdued highs and the Hewlett to have more subdued lower frequencies (Studio v Live or Carnagy Hall acoustic?).
But, one must consider that I voiced my system for smooth and clear, but not dominating, highs. Maybe that is the reason for my impression of these two recordings. If you don't hear the sonic's w/Kissin as I described, big, warm, w/out HF emphasis, then we may just be talking about the inadequacies of my system/setup or my aging ears. :-)