Listen, don't take it the wrong way ... but I have absolutely no doubt what-so-ever that the Foster tweeter produces large amounts of distortion. I'm leaving in a few minutes, driving 10 hours for Minneapolis or I'd search for some numbers on that driver. I remember some of the guys on diyaudio.com buying them and testing. It is the cheapest such driver around, I've seen it before for $40/pair. And when I talk about testing I don't mean listening, I mean using a measurement setup to see linear and nonlinear distortion, frequency response on and off axis, etc. I have a test setup myself, so I could test one of the Fosters if I had one.
My point isn't so specifically about that driver, although I do remember reading about someone testing one. Once you've tested and seen enough driver tests you can pretty accurately guess what the performance will be like based on the construction of any driver. There can be exceptions to general trends of course, but I've never seen a Kapton Ribbon that performs decent enough for me to consider using it in my own DIY designs. Dome tweeters simply vastly outperform them, at less cost.
About distortion, again don't take this the wrong way but I doubt you'd even realize if that were what you were hearing. It really is hard to hear distortion, especially even order harmonics, and in fact that's a reason why many people like ribbons is because of the distortion - it can be a pleasing effect, often given terms like "air", "sparkle", and "detail". And it takes a VERY experienced ear to tell the difference between REAL detail and detail due to distortion. I even get duped by it at times myself. If you do like the sound, there's nothing wrong with that ... people like tubes too, because of the way they can color the sound, not because of their faithful and accurate reproduction.
Specific to the HTD tower, there are some serious problem with it's implementation. Mainly, why the heck did they turn the ribbon on it's side? That tweeter has horrible off axis vertical response, turning it on it's side will just give you horrible off axis horizontal response. Also they could have put the dome mid much closer to the tweeter. The fact that they have to cross that tweeter over so high (4000hz) is due to the fact that it can't produce anything under that point without ridiculous distortion, but leads to a whole string of problems. Because of the high cross point, you're going to have serious lobing problems enhanced by the large Center to Center distance between the tweet and mid.
I don't want to make a big deal out of picking those speakers apart. And like I said, there's nothing wrong with liking them. It's not uncommon at all for a speaker to sound decent, but measure badly ... and I'd suspect that's the case here. I've never heard them, I'm only looking at it from a design and engineering standpoint - but honestly, from that standpoint they look like something tossed together buy guys without much knowledge or experience.