Norman --
You mention that this piano is in a lobby. Yeah, the acoustics are going to be horrible and a live acoustic piano is the wrong instrument for the space. You'll never get the piano acoustically quiet enough to satisfy the needs of the room and still have it sound acceptably like a piano. On the other hand, some of the suggestions here could work in the home. I put 3" foam in the back of my Yamaha U1 and it knocked it down quite a bit. But again, if it's a large live room you're never really going to get it quiet enough to hold a conversation in the same room.
I service two locations where the piano is in a large live environment. In both locations the owners chose a Yamaha Disklavier so that they would have a real piano but also be able to play it super quiet using the "quiet" mode, which blocks the hammers from actually touching the strings and, instead, provides a Yamaha CF9 concert grand sample over speakers to replace it. You can get it almost silent quiet and it still sounds decent. However, replacing that Steinway is obviously not going to be an option.
But, since this is a Steinway, rather than go to extremes that are probably not going to look attractive, and probably not satisfy the needs of the room, I'd like to recommend an add on. Check out the QuietTime system by PianoDisc.
https://pianodisc.com/quiettime/ It's a relatively inexpensive add-on that is easy to install, non-destructive to the piano, and functionally it delivers a system that can be played both acoustically and digitally over headphones. Only thing I'm not certain about is whether it can be used with speakers. My guess is that if you plug in powered speakers to the headphone jack it will work just fine. Might be worth contacting PianoDisc to find out.
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Geoff Sykes, RPT
Los Angeles CA
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