O/B Speakers

Anyone here using o/b speakers? If so which ones, how much was the cost? What is their sensitivty? What amp are you using with them, are they passive or active?
I built a pair of DIYs.The woofer is the biggest question to solve.I heard the Peerless XLS work very well in this type configuration.I used a plate-amp for my bottom end,to make it an easier load for my PL2.Be sure to address phase-alignment,as trying to correct after-the-fact is difficult.
Possibly making it a multi-woofer two-way is a good way to go,as it will reduce any over-excursion to these drivers and simplifies the crossover-not to mention allowing you to design in a higher impedance for tubes.