reproducing Roland's ultimate piano

I have Roland RD300GX model. I bought it because I liked the sound of Ultimate piano (#2 in the built-in "piano" bank). I tried all instruments in a store - Yamaha's, Corg's, etc. - and couldn't find anything that sounds like Ultimate. I have difficulty explaining why I like it, in technical terms. It sounds "warm" to me, while other options sound a bit (sometimes a lot) "electric". Can someone please explain it me what is special about this sound, in terms of physical parameters, so I could try to reproduce it on Pianoteq (I don't think Roland used any specific samples for Ultimate - probably, they just tweaked whatever they had).