Need opinions on super tweeters as an add-on

Tbg some ribbons are both tweeters and super tweeters, many can cover hi-frequincys and ultra sonics. Super tweeters are best matched to ea loudspeaker. The one super tweeter for all loudspeakers is hit or miss when in real use. Many loudspeakers on the market claim frequincy extention to 20khz but most do so at reduced SPL on axis and off axis greatly reduced SPL. Since many super tweeters are designed to cover arround 15khz-35khz+ or at 25khz-up. With loudspeakers where frequincy is reduced on axis and off before 20khz, do better when added super tweeters running lower in frequincy. This does not mean that added tweeter cant be ultra sonic. I know of a fountek ribbon that can cover 1500khz-40khz at near 102db 8ohm. Many compression tweeters like Fostex t500amk2 also cover hi-frequincy and ultra sonic. A loudspeaker that uses a tweeter than covers some ultra sonic might benifit from a super tweeter with higher crossover. But than again might not benifit as much if at all. I feel extention to 40khz is about all thats needed. But with so many loudspeakers with domes[most all domes are not flat to or past 20khz on axis] or many 2 way designs[tweeter needs to cover upper mid range thus limited hi-frequincy extention] many of these designs since they roll off trebile benifit from adding tweeters with ultrasonic extention. If you own a ribbon loudspeaker or hiquality compression tweeters you will gain little to nothing for frequincys are already reproduced. If you own a 2 way,dome tweeter,fullrange driver,vintage horns, electrostatics super tweeters might be worth a listen and you can DIY a pair.Check out madisound.