I won't say the Paradigm Servo 15 I use is the best thing out there but I will second Beam's post and add that it is damn good for the dough. Maybe the best sounding serious sub for the money. I don't take any of the "little wonder" subs very seriously. Once you get past the gee-whiz factor they just don't sound as good. Better yet two can be purchased for the price of things like the Muse 18B or Aerial Sub and in my experience two good subs sound better than one great sub when placed carefully.
You can't really expect much from the high-pass section of any sub crossover so if you are really critical about transparency you should run your main speakers full range. You lose any overall increase in system dynamics but you don't mess up everything else just for a few extra dbs of headroom. Using the adjustable low-pass on the crossover, bring the sub(s) in where the mains fade out. Variable phase is a very usefull feature also allowing you to better match the sub to the mains. I've been using Servo 15s with a pair of Paradigm's X30 crossover boxes with Merlin VSMs in a very high end system to great effect for a number of years.
You can't really expect much from the high-pass section of any sub crossover so if you are really critical about transparency you should run your main speakers full range. You lose any overall increase in system dynamics but you don't mess up everything else just for a few extra dbs of headroom. Using the adjustable low-pass on the crossover, bring the sub(s) in where the mains fade out. Variable phase is a very usefull feature also allowing you to better match the sub to the mains. I've been using Servo 15s with a pair of Paradigm's X30 crossover boxes with Merlin VSMs in a very high end system to great effect for a number of years.