Best Loudspeakers for Rich Timbre?

The radial cone in Harbeth speakers is quite rigid or pistonic.

Richard should not use a 4.5 inch cone at 5 KHz - it will beam severely. But the idea of pistonic is good provided the material is intrinsically damped. 

Watch the two videos on this page if you want to learn the difference between vacuum formed polypropylene and Harbeth injection molding.

https://www.harbeth.co.uk/usergroup/forum/the-science-of-audio/speaker-design/2215-real-world-drive-unit-and-crossover-issues/page2

Alan Shaw clearly has put a lot of effort into that mid range cone. It is superior to the large majority of designs. Like the ATC mid range - Harbeth have rightly acquired a reputation for excellent mid range quality.

It is neither rigid, (nor pistonic), if compared to CF, aluminum, or ceramic cones, all you have to do is touch one (You can actually see that in the measurements as well, if you know what to look for). I have seen a dealer stand on a Magico 6" cone, that is rigid.

Yes aluminium and ceramic are nearly perfectly rigid and pistonic but they Rrriiiiinnnnnnggggg like a bell and are terrible materials for driver cones. The Rrrrrriiiiiiiiiiiinnnnnnnnnnnnngggggggggiiiiiiiiiiiiiiiiinnnnnnnnnnnnnngggggggggg is what I hear from these designs. Terrible waterfall plots in most cases. Magnesium is better. CF is pretty good. What Richard uses will also be good - carbon fiber and balsa - both these materials will dissipate internal energy.

The second video on the page I linked above shows that the Harbeth radial cone supports a 4.5 Kg weight - this is pretty good rigidity!!

A good driver balances rigidity with intrinsic internal damping. The use of a large diameter voice coil can also make a huge difference instead of the typical 1 or 1 1/2 inch consumer audio woofer voice coil (which offers little support to the diaphragm and places a lot of stress at the apex of the cone)

That is why the cones should be sandwiched, creating a constrained layer damping mechanism. See Magico and Zellaton. Focal does that as well, but uses fiberglass, somewhat less effective.
Magico uses 3" and 5" voice coil, btw.

My original Legacy Focus speakers use cheap paper woofers (3 per speaker) and kevlar mid-ranges (2 per speaker).  Legacy upgraded the cones and doubled the magnets in these speakers as Focus HD and SE.  Neither of the two newer speakers sound as musical as my old fashion, cheap cone material speakers.  How much better would my Focus speakers sound with Harbeth cone drivers?  Apparently, a lot better.   But to date, my Focus speakers are overall more enjoyable than Harbeth 40.2s I've heard four or five times (sounding very nice too).