I suspect they probably behave a lot like my Harbeths, which at 86db/w would on paper seem well enough driven for domestic purposes by 60-80w amps, but in my experience it just doesn’t work.
They only ever come to life when I strap them to something of at least 150w
That is an observation I am only just beginning to get my head around. The BC1’s have cross-overs at 3khz and 13khz and the big pitch at the time they were released was the main driver and it’s compliance, thus they are pretty sensitive. But the 303 does not seem to have the ability to control free movement of the cone when the signal reduces resulting in a element of One - note Bass, rolling off quickly.
With the 405, the cones don’t seem to have the same freedom to do what they want. The result is the Bass is deeper and tighter, and any boxy-ness all but disappears. Bass is much more natural and mid-range appears to have more clarity. They are a much better speaker with the 405 than they ever were with the 303. They were good, but now seem to have improved dramatically.