We had some monitors like that here, they were a similar 87dB and with my 20W Raven Blackhawk had no problem driving them satisfyingly loud in my 17x24x9 room. The guy who brought them had also brought his 100+ watt SS amp. We were all surprised how those inefficient power hogs came alive with the Blackhawk.
It is good you are figuring out something I have used as a speaker buying guide all my life- avoid low sensitivity speakers, they limit you on amps. But unless your room is huge or you rock out a lot (not likely, Harbeth after all) then you will be fine with any tube amp 20W or more.