Having compared the RD series drivers to a line of Neo's there are a lot of good reasons to go with the line of Neo's.
For one, they sound a lot better. Then there is the much higher sensitivity and power handling. The Neo's also don't have any resonance problems, and they extend much higher on the top end.
The Neo's won't play quite as low, but pushing the RD driver that low doesn't sound realistic either. While it will play low, it won't do so with much output capability. It just has no ability to move any air in the lower ranges. I'd use a dynamic driver below 800Hz.
For example, the hit of a snare drum. Through the RD driver crossed over real low, it sounds like a snare drum, but doesn't have the feel of a snare drum. There is no sense of the impact. It is like listening to it on head phones.
Crossing in the 300Hz range puts the crossover point in the heart of the mid range and is not a good place to be splitting vocals into dissimilar drivers.
As far as making the line of Neo's longer, the more you use the higher the sensitivity (depending on grouping numbers), the higher the power handling, and the less load there is on each driver (lower distortion). Plus, the more of them you use the more gain you get in the lower region and the lower you can cross them. I have used 6, 12, 18, 20, and 21 units in a line. The larger numbers sounded better.
