Home demo. ProAc D48R and Spendor D9

Understood. Unfortunately, the space I have is not very large - about 23 square meters with a height of about 2.3 meters. I can get about 0.5-0.6 meters between the back of the speakers and the (long) wall behind them with the speakers spaced about 2 meters apart (inner edge to inner edge) and about 3 meters between me and the front baffle of each speaker.

I think the mistake you are making is you are considering the ProAc D30 and Spendor D7 as small. And it is a big mistake. When I auditioned these 2 loudspeakers I was amazed by how much "larger" they sound compared to their size. Both these speakers produced low range frequencies with amazing speed and definition.The bass did not sound like "column of air" being thrown out to "sound like bass", but real low bass. With the way you mention about the positioning of loudspeaker in your room, these can go down to 30Hz without much struggle. With your configuration the D48R and D9 will overload your room and you will regret the purchase even though you may be able to afford them.

To further confuse things, I’ve read somebody prefered the mids of the D30RS to those of the D48R, stating that the midbass driver is better in the D30RS.

Like I said, the D48R needs breathing room. If they don't get it, yes, the midrange can become muddy. In this case one should not blame the speakers, but rather blame their choice of speaker knowing their room limitation.
Based on your "music diet", I suggest that you listen to some Harbeths and Sterling? (not sure, looked like a box like Harbeth, but many screws on the front when you remove the baffle; heard at AXPONA 2017).Looks like you are based somewhere in Europe, where ProAc, Spendor, Harbeth are much cheaper than the US.

I did listen to Harbeth 30.2, but it was with an amplifier I believe was not up to the task, a Sugden A21SE that seemed underpowered to me and an accompanying friend. I want something more dynamic and with much better low frequency extension. I liked the Spendor SP2/3R2 much more (but with a much stronger and expensive Densen amplifier, to be fair).
Would the big bass of the bigger speakers also muddy the midrange at moderate to low volume levels?

ThaNks again for your assistance!

Would the big bass of the bigger speakers also muddy the midrange at moderate to low volume levels?

In a small room there is a potential that bass would muddy the midrange at moderate volume.The thing is, the ProAc D48R is very easy to drive. I posted my experience from this weekend where I brought the $89/pair Pioneer SP-BS22-LR speakers into my main listening room to see how they fair with good ancillaries. While they punched too much above their price range, on similar CDs, I had to raise the volume to 11'o clock position for them, compared to somewhere between 9-10 o'clock position for the ProAcs. I believe, the D9s are also similarly sensitive.But if your room has an "open" layout, the bigger speakers might be alright. This means, you don't have a closed room, but your audio room opens to a kitchen or living room or some other room. We have one gentleman on this site who owns a D40R and has been very happy with them as he has an open layout.

I’m afraid it’s a closed room. Not sure what is the significance in this context of the fact that the D48R is easy to drive.

Isn’t it a matter of too much bass extension, exciting some room modes (say, below 30 Hz) that other less extended speakers wouldn’t touch? Because if this is all the problem and, let’s say, D30RS extends to 30 Hz and D48R extends to 20 Hz, then the problem would only be apparent with recordings with significant content between 20 and 30 Hz - and there’s not a lot of it in the music I’m listening except for the symphonic (occasionally) and organ music. I have a feeling that I’m on thin ice here...

Also, can this "too big a speaker for the room" problem be addressed with some room treatment? All this guesswork is worrying me as this is not the kind of expense I’m likely to be able to repeat in the foreseeable future. Some people recommend mini monitors like Dynaudio Special 40 for a space like mine, others are cramming Harbeth 40.2 and the like and declare themselves very satisfied...

Finally, to return to my original question, between D30RS and D7 what would you choose for piano reproduction and why? I care a lot about timbre and especially that tactile, percussive quality of the upper range of the piano I tried to described above.

I currently own the D30RS and all I can say that they exceed my expectations by far first they sounded a little bit blur,weak dynamics and laid back after a very long break-in they are now wonderful their ribbon twitters give very natural life like highs,the mid is superb detailed and the bass is powerful , punchy and accurate on of the best I ever heard .the soundstage is huge and the sound is very natural . I think the D30RS will work very well in your size space without overhang bass that the D48 might do. Didn't hear the D7  however one of my friends who considered to buy them heard them at high end show and was very disappointed.