I suppose that tendency for sibilance in the upper mids is another way of describing what I heard in the 30.2’s (and also in some 40.2’s I’ve heard in a friend’s system). But sibilance is too hard a word, I think. A shade of sibilance, No such thing in the Classic series Spendors I’ve heard ( 3/5 - those before the R2 series I think - , SP 2/3R2, Classic 100) at all, though.
I know exactly the “shade of sibilance” you mentioned. Of all the Harbeths, I find it most apparent in the C7ES3s and SHL5+. Naturally, one would presume it’s the result of their metal tweeters in those models. However, I also heard it to a lesser extent in the 30.1s, and least in the 40.2s and P3ESRs. This leads me to hypothesize that the 8” Radial cones are exhibiting more audible breakup, since they are crossed over relatively high.
To the OP:
The common (perhaps unfortunate) misconception is that world-class midrange performance is the exclusive jurisdiction of these BBC inspired brands and their trademark polypropylene cones. The reality is there are many brands that match and even exceed the midrange performance. Good examples are some of the Joseph Audio and Tyler Acoustic models that employ magnesium Seas Excel drivers. Those drivers can actually make a Harbeth or Spendor sound a little distorted and homogenous by comparison. The truth is that that these BBC-derived brands are all employing drivers that are equivalent in quality and performance to <$100 off-shelf models from Seas, Scanspeak and SB Acoustics. I would pit my Tyler Taylo Ref monitors against any of the aforementioned Brit boxes in terms of all freqs above ≈60Hz. As much as I enjoy and respect these old guard brands (Harbeth/Spendor/Graham et al), they simply do not keep up with speakers equipped with top-shelf parts and drivers IME. I replaced the OEM drivers in my Stirling Broadcast monitors with off-shelf Seas models and they now perform in an entirely different league. The Brit boxes are good among dealer hawked speakers, but their hype (and prices) have become misaligned with their performance in recent years, in my opinion, FWIW.