Looking for input: Best material for mid range cone

@helomech,

No, like I said in another post, I didn't waste my time listening to every single $4000 speaker produced by every manufacturer on the planet. Harbeth, to the best of my understanding, basically produces highly refined BBC monitor type speakers. I'm not at all surprised Harbeth chooses to use polypropylene cones because a primary characteristic of BBC monitors is the classic British dark tone. I wasn't particularly taken with B&W because those Kevlar cones tend to be darker too. From everything I've ever read about polypropylene cones, and specifically Harbeth, I didn't bother wasting my time hunting down a pair to listen to because nothing suggested to me that they'd be to my taste. I fully realize some people would sell a testicle for a nice pair, but I'm not one of them. 

Polypropylene not so good above 1Khz.

Basically most speakers have too large a mid range for the bandwidth they carry (results in beaming)

Stiffer materials can measure extremely well (detailed) and have wide bandwidth often extending higher in frequency but tend to have resonances or ringing (poor timbre hashy sound).

Softer materials with weave or fabric or pulp paper mash can be great because of internal damping (get very clean background or exceedingly good timbre)however they suffer from linearity issues (limited bandwidth due to breakup)

In a three way softer materials for cones can work extremely well. In a two way stiffer material is often the best compromise to coverthe audible range in only two drivers.

planar speakers are favored by most of cats.

Where do you guys come up with these blanket claims? "Polypropylene not so good above 1Khz." Based on what? My speakers have a 7" plastic cone and play up to 3kHz with hardly a lick of distortion. They play much cleaner than numerous other speakers I've heard with exotic cones. There's good and bad examples of almost every speaker diaphragm material. I've heard aramid cones that sounded like complete garbage, and some that sound great, the same for paper, plastic, titanium, and aluminum. I've owned speakers with plastic tweeters (oh no! Over 1khz!) that blew away some well regarded silk domes.

To my next point: size matters! Any loudspeaker engineer worth his salt can tell you that a material's performance can heavily depend on the size of the diaphragm, the frequency range it's asked to cover, and the SPLs it's asked to play. It's true that large plastic cones CAN struggle with higher freqs, while paper cones exhibit their best virtues in the larger sizes. I haven't heard many good sounding paper tweeters, however, like with most materials, I'm sure there's exceptions out there.

OP, 
It really comes down to personal preference and the quality of design. Speaker sound is the most subjective aspect of a system. One person's euphoric midrange will make another's ears bleed.

I have owned rega rs5, which utilize paper cone for both midrange (5") and woofers (7").  The midrange on these speakers was outstanding - natural and transparent, it is really their big selling point.  I have never got the same with kevlar (B&Ws at twice the price, and Wharfedale at equivalent price).
The new line rx also employs doped paper cones... Haven't heard them; I am not sure how they compare to the rs.