I've been looking forward to a detailed review of the C-note for quite a while now. Thanks Amirm! (dang, you test alot of speakers!)
This is pretty much what I was expecting.
The 700 Hz peak is most likely a resonance, or combination of resonances. I'm not sure if Winkleswisard added any damping inside the enclosure, but I'm guessing not (no problem, I agree with testing the kit as it comes). Internally the cabinet is 10" high, which is just about 1/2 the wavelength of 700 Hz. I assume that is the majority of the issue, a standing wave can add a couple dB peak to vented enclosures. I recommend significant damping in any enclosure to keep that under control (It is not included in the kit just to keep cost down, $100 was a hard target to hit, lol). Also, the side panels have a resonance in that area, so that is adding to it a bit (just a little), adding a brace tying the sides together would help a little. Port resonance might add to it a little, but 700 Hz just seems higher than I would expect for a significant port resonance. You could stuff a sock in it, but you'll kill the bass... Nobody wants that.
I doubt the peak has much to do with a leak around the tweeter. It would be easy to check for that, just play a tone around the tuning frequency. About 40 Hz. You'll hear significant chuffing around the tweeter/woofer overlap if there is a leak. Pressure in the enclosure is highest at tuning, air will get out anywhere is can.
Speaking about the bass response, I'm glad that you are happy with it. The C-note is designed to be used near a wall, so that will add quite a bit of boost if you are looking for flatter low end response. But in my experience, extended low end is better than flatter low end, if you have to choose between the two (small speakers). Your brain is good at filling in dips in low frequenies, but there has to be something there to begin with. So, the C-note enclosure is not exactly optimum, more of an extended bass shelf alignment (EBS). Plus, that keeps excursion under control for better power handling (like amirm said, "I could barely see it moving"). I'm not a fan of speakers tuned to 50 Hz or higher unless it is specifically designed to be used in a system with a subwoofer. That is just a recipe for over-excursion disaster... Just go sealed at that point.
About the higher distortion at the crossover. Yeah, the 3,000 crossover is really pushing the limits of that little tweeter. It was a compromise to get better vertical dispersion. There were many crossover designs, some had crazy flat response, but had many more parts ($$$) and in side by side listening tests the difference was negligible.
The C-note was fun to design... An abosute exercise in simplified and getting that most bang for your buck.
Oh, about the gorilla glue crossover thing. We send a lot of speaker to trade shows and crossover parts alway break loose in shipping (inductors mostly). No matter how well we mounted them (zip ties, screws, even epoxy), the shipping companies alway found a way. Completely embedding the crossovers in Gorilla glue seems to have fixed the issue. I even dropped a Blast Box down a flight of stairs with no issue (well, with the crossover at least, lol). Plus, it...uh... helps with microphonics? Yeah, that's it, microphonics!
Thanks again for the review Amirm! I'm glad you were satisfied with it.