Is it that far-fetched of an idea to take a disc shape of foam, similar in density to what used to be used on some manufacturer's speaker grills in the 70's and 80's, and just cover the tweeter to reduce the brightness, of course being careful not to physically touch the tweater and afix to the baffle with tape??
You don't mention whether your shrillness problems are at all volumes or just loud volumes. The following may be a bit of an aside. Here's what I have done on my B&W 805S, standmounts, experimenting when I have played select CD rock recordings that are just too shrill when playing them rocking loud, examples are My Bloody Valentine "Loveless" or Sonic Youth's "Daydream Nation", both great records and really shine when played loud, IMO, except for the shrillness. I use the foam cylinders (approx 2" diameter, 3" in height) that came from B&W with the speakers which are provided to optionally stuff in the reflex port in situtations where the speaker is to boomy, BUT instead use them on the tweeter, holding them in place over the front of tweeter (co-axial with the tweeter) with a small rubber band. With the B&W tweeter you can use the rubber band since the tweeter is in a small separate housing on top of the cabinet as most well know. It works from me and can be put in place in seconds. I notice a slight degradation in imaging but would rather live with that then having my ears be victim to the shrill guitar notes within these recordings yet enjoy the benefits of loud volume in the bass and mids which apparently my ears are better adapted.
You don't mention whether your shrillness problems are at all volumes or just loud volumes. The following may be a bit of an aside. Here's what I have done on my B&W 805S, standmounts, experimenting when I have played select CD rock recordings that are just too shrill when playing them rocking loud, examples are My Bloody Valentine "Loveless" or Sonic Youth's "Daydream Nation", both great records and really shine when played loud, IMO, except for the shrillness. I use the foam cylinders (approx 2" diameter, 3" in height) that came from B&W with the speakers which are provided to optionally stuff in the reflex port in situtations where the speaker is to boomy, BUT instead use them on the tweeter, holding them in place over the front of tweeter (co-axial with the tweeter) with a small rubber band. With the B&W tweeter you can use the rubber band since the tweeter is in a small separate housing on top of the cabinet as most well know. It works from me and can be put in place in seconds. I notice a slight degradation in imaging but would rather live with that then having my ears be victim to the shrill guitar notes within these recordings yet enjoy the benefits of loud volume in the bass and mids which apparently my ears are better adapted.