I think this is a little misguided. You more than likely don't even hear 20k. That's where a lot of manufacturers just stop referencing their gear to. When you see something that says 20hz-20k it doesn't mean it stops at either end of those, it means its full-range. More than like the tweeters in the tannoys go to 30k. But other than the advantage of not messing up harmonic content that could mess up what you do hear, its pointless. Especially if you're not sending much above 20k to the monitors in the first place. I only hear up to 18k in my good ear, and I'm 23 with good ears. 20k is usually only possible with kids around 12 years old when your ears are prime and haven't been abused by loud concerts and explosions. I don't personally know anyone my age or older who can go to 20k. I'm sure there are a few select lucky bastards, but I don't know any of them.

I suspect Tannoy spent a bunch of money to develop those speakers, including addressing phase alignment between the woofer and tweeter at the crossover point. If you have the R&D capacity to get the replacement tweeters to align properly then go for it.

I also suspect that the "dark" sound of the monitors has nothing to do with lack of response above 20kHz and everything to do with the response below 20kHz.