As mentioned earlier efficient horns with a large woofer preferably of 15 in.
work well at low volume. Klipsch Cornwall lV may fit the bill. A quality amp where, again as mentioned, the first couple watts are clean, such as a number of Class A
Nelson Pass First Watt models. Think F3, F5, SIT 3 or 4 or new(er) F8. Add a quality tube preamp and I think you have the deep, full room filling low volume sound you’re hoping to find.
Best
Rich