Hi Yamatai,
The Sapphire line is a great lineup to delve into the world of open baffle. Spatial is planning on re-releasing the M4 as the tweeter that made that design iconic is back in production. We currently have a large order in for the next batch, although at this time, we cannot put a timetable on when the speaker will be available. If your looking for something more sensitive spatial is also working on an S5 which is a partially active speaker in our current S series lineup. It will hit mid 90’s efficiency and feature the same soft dome as the S series lineup.
I personally own the M4 Sapphire, and love them. If you don’t listen at loud volumes I think you could get by with the 88 DB efficiency as my amplifier is only about 40 Watts and it’s more than plenty. If you need the extra efficiency, I would recommend looking for an M3 on the used market.