How does the 18W/8531G00 sound compared to the 21W/8554-00?

I appreciate this is a question that depends on a whole load of "what if" scenarios, but has anyone experienced Scan Speak's paper 18W/8531G00 and Kevlar 21W/8554-00, and might be able to offer advice on the sound (as compared to one another)?

I have a pair of 45L ported speakers (Wilmslow Audio Nemesis) using the 21W/8554-00 with the D2905/9500 treble, and really enjoy the sound they produce. Unfortunately one of the 8554 drivers has died, and I've so far been unable to source a replacement.

As I have the D2905/9500 tweeters I've been thinking about building Troels Gravesen's SP95 design (SP95) as I understand the 18W/8531G00 is highly rated.

I assume an 18W wouldn't match the larger 21W for bass output in the lowest frequencies, but I cross over the speakers with a pair of sealed subwoofers so that shouldn't be a concern.

Any thoughts/guidance/experience appreciated.