Firstly, thank you for the excellent service!
I wanted to replace a pair of old 92mm faceplate Scan-speak D2008/8203 tweeters I've been using with KEF B100B bass driver for many years. The replacement D2008/8511(now discontinued) would 'drop in', although isn't a perfect replacement efficiency or sound wise (the 8203 was heavily doped). The wonderful Hiquphon OW1/92 would be first choice but sadly out of my price range!
The SB21RDC-C000-4 required a little modification to the cabinet, simply to accommodate the connectors, and has four screw holes rather than three - but fits flush with the baffle.
I've had to re-work the crossover somewhat (this is a 4ohm tweeter), but the results are very promising using a third-order high-pass with an l-pad (1.5/2.7 ohm based on the 3 ohm DCR). Currently using a 5.6 and 16 microfarad capacitors and a 0.125mH inductor, which seems to blend rather well. I'm sure I'll tinker more, but there's really nothing to dislike as it is.
Thankyou for your review Peter!
Will & Caroline