I work for Danny, as I'm also really familiar with a lot of our speakers, (just so that's stated up front) but I welcome anyone else's thoughts as well.

1. In terms of dynamics the Bully and NX-Otica/Treme are the most similar, where the NX series wins is in clarity/detail and overall soundstage depth/layering as the NX-series really "disappears". The NX-studio isn't a great comparison as it's much smaller and only 87dB efficient vs 94db of the other models, but it's soundstage performance is overall quite similar to the rest of the NX-series, you just lose the dynamics of the other models. Where the bully wins IMO, is in terms of fun/engagement, as the soundstage is more "forward" it's killer for more "in your face" type music (Rock/Metal etc.) But that's not to say it cant be refined or articulate when it need to be, it just wont be to quite the same level as the NX towers.
2. Since they're all very similar in terms of sensitivity, such an amp will do a great job with any of them. The Bully's do offer a lot more flexibility in terms of placement as they are a sealed design with a powered subwoofer controlling the bottom end. While the NX-series needs to be pulled away from the front/side walls. That said with any of our models, the further you can get them in the room the better.
3. Compared to the majority of the X-series, the Bully's are more fun/engaging thanks to the higher sensitivity. I'd day that most of the X-series does disappear better thanks to the narrower baffle, especially models like the X-Statik which have open-baffle midrange. The Brute is a more direct comparison and they're quite similar overall, especially in terms of soundstage performance, and aside from the 4db difference in sensitivity, but having a servo controlled 12" sub in the Bully over the passive 12" poly cone of the Brute really lends itself well to more speed and impact, especially down low.
4. The Next plan is the "Beast," which the current plan is to use 2x 6.5" paper cone mids with 2x 12" sealed subs down below, using a more powerful amplifier. But we're not ready to start on prototyping that out just yet. We don't have plans to change the tweeter.
5. It really depends on what sort of sound/performance you're after. How far can you pull the speakers out into the room? Can/do you want subs? (and can they also be pulled out from the wall?)
If you want a speaker that completely disappears and is articulate and refined with a deep layered soundstage, and you can pull the speakers out into the room, look at the NX-series.
If you want a good all-round performance on a budget, look at the X-series, if you can pull them from the wall, look at the X-Statik in particular.
If you want something big and engaging, then I would go with the Old-school series.