Sorry to be having this conversation with myself in public, but I think I've fixed the problem I described in the previous post and thought the nature of the fix might (just) be of some interest to others.
The primary speaker setup I was using was a 3-way system (bass, mid and treble speaker drivers) with a pair of ATC SM75-150 3-inch domes handling the mid frequencies. These are interesting drivers, generally well thought of (eg http://studio-hifi.com/images/ATC75-150...agby.pdf), but with a very particular character which I'd never been entirely happy with in this particular setup. What I've now done is to add a pair of SEAS M15CH001 mid-range drivers (now superceded by the M15CH002: http://www.seas.no/index.php?option=com...temid=359) to handle the lower mid frequencies (225Hz-1.1Khz in my setup), leaving the ATCs to just deal with the upper mid range (1.1 - 6KHz). The result is little short of magical, particularly with regard to Pianoteq, and particularly with regard to my problem with the D4 area on the Steinway model D. This area of this piano now has the most beautiful bell-like and slightly breathy singing tone making it one of the most attractive regions of the whole keyboard. What a transformation!
It's the ATC unit that is making this sound and I suspect it would be difficult to get it with any standard cone-based mid-range unit. They just sound very different - seemingly much less detailed. It may be that in the future we will start to move towards different types of transducer which use piano-like wooden soundboards rather than the cones or domes of standard loudspeakers to produce the sound (eg as here https://forum.modartt.com/viewtopic.php?id=7045). I can only imagine that these will sound very different again and that they may well be much better. But in the meantime I'm now very happy with the setup I've got and I'm sorry to have given the impression in my previous post that there was something wrong with the PT D4 model.
Ian.
N1X - PT Pro - Linux
