I really have no personal experience with audio rooms that large. But as long as you listen no more than 10 or 12 feet away, I imagine that the SLH5 would work there. Certainly the Monitor 40 series would. With any Harbeth speaker, room placement experimentation may be necessary to tame the midbass. More recent Harbeths are much superior in this respect to the original SLH5 or M40. In any case, be prepared to use electronic equalization to tame the midbass and perhaps correct issues around 1 kHz with any Harbeth speakers. I've always needed and used electronic EQ with my M40, M40.1, and M40.2 speakers, even after careful room placement.
Other speakers I have extended personal experience with which I think might work well in your large room are the Dutch & Dutch 8c and the Sanders 10e. There are threads in Tom's Corner about each of those. Both of those come with built-in electronic equalization capability and will certainly play loud enough without strain for a large room if you listen 10 to 12 feet away.
Other speakers I have extended personal experience with which I think might work well in your large room are the Dutch & Dutch 8c and the Sanders 10e. There are threads in Tom's Corner about each of those. Both of those come with built-in electronic equalization capability and will certainly play loud enough without strain for a large room if you listen 10 to 12 feet away.