Don't know them, but personnaly I wouldn't be concerned about 18 kHz. For one thing, it's a log scale so there's not much between 18 and 20 kHz (if you can still hear it at your or my age). For another, my own speakers only go up to 17 kHz at -3 dB! It doesn't sound like I'm missing a lot to me. But in this age of super-tweeters, who really knows? Maybe in a few years there's be a supertweeter crossover mod for my speakers available! :-)