I am sure if you dig deep enough, all of them likely have things that either clean up or extend response integral to the amp, so that is a non-issue in my book. I have an Empire, so I can say from experience, how it is done doesn't matter. The main performance for their subs, and where the vast majority of the sound is, is higher up in the range.
Yes, servo is a great thing. I've heard a sealed Rythmik and it is a solid performer. I also previously used one of their amps for a sub I had and I have no problem recommending their products at all. Prior to buying the Empire, I was considering building one of the GR/Rythmik (GR Research driver w/Rythmik servo amp) subs.