OMG dude... I LOVE the tone of Steinways! Nice lushness to the sound, a lot more detail than I've gotten out of Steinways with lesser pres. Here's what I'm hearing that may be what's bothering you, though.
Definitely some warm smoothness to the tone... say, 200 Hz and below. There's also a bit of top-end sparkle (could be from the MP3 encoding) that sounds a little out of place. I'm hearing a relaxed midrange sound. So, try this:
Use some low shelf at 120 Hz or so, nice, long slope. Drop the lows about 3 dB or whatever sounds good to you. Or try a 1-2 octave wide bell dip centered around 100-200 Hz. Again, don't take much out.
Roll off the highs at about 4k... Put a little, wide bump in the mids about 800-1000 Hz.
If you re-record... Pull the mics away a bit. While she's playing, move the bass-side mic around while listening to your cans and find a spot that's not as boomy (there is definitely a bass sweet spot in those!). You'll likely only have to move it a few inches.
These are just suggestions, and I'm totally pulling this out of my rear since I didn't actually try these suggestions.
But there's definitely a little extra boom, and a little extra sparkle around 4k, and a relaxed mid. If you reverse those, you might get the sound you're looking for. I don't hear any phase problems as such... There is a wide stereo spread, though. Don't take this as criticism... Overall, I think you've captured a lovely sound!
But you said the artist likes the mellow sound of this piano, and didn't like the Bechstein. The scooped mids in the sound of the piano could turn out to be a good thing for you if you're going to have vocals and/or some guitars and banjos (don't know what all goes into neo-folk). As far as feeling right... I'm not a piano player, but I've pounded on all sorts of pianos... And the Steinways definitely do have a particular "feel" to the key action that other pianos don't. I've found that whatever crap I'm pounding out on a Steinway always sounds better than on, say, a Kawai... It is very easy to control dynamics on a Steinway (Kawais are just plain loud), and the ease of playing it helped me relax a little bit. So, yeah, if she can work her magic on the Steinway, that's the piano to use.