Supertweeters - please advice which one

Why not buy speakers with super tweeters already in them instead.The engineers have worried about the crossover for you.
Further I find integrating seperate speakers is not as easy as it appears.

I would completely agree with Mechans. I tried the ERT Supertweeter from AV123 which is relatively inexpensive and was recommended to me by another member. While it was additive in the upper range it was also subtractive. It took away the balance the main speakers originally possessed. In the end, I sold the ERT's.

I have a pair of LCY Super tweets ($500/pr) that I use occasionally. I have found them easy to integrate, you just wire them directly to your binding posts and they sound fine. I have also used them with an eq but found that wasn't necessarry. I find them useful at low listening levels, not so much as the spl's increase.

At 99DB 1 watt a LCY at only 92DB 1 watt wont match. None of the available supertweeters are this hi-eff. And all mass produced have crossovers ball parked so none will exact match your loudspeakers. So you need to DIY or have a custom super tweeter built. I suggest fostex t500amk2, goto sg06, fountek neo pro 5i it does roll off a bit in ultra sonics. KCS can custom match these drivers or go with the goto it only needs a capacitor to match the supravox.