These are PD-2150 - 21" professional drivers from England have great specs to build pair of subwoofers which can go very low.
The efficiency is 98 db meaning they don't require a lot of power. The Fs = 29.5 Hz. Impedance - 8 Ohm.
I ran them in the corners of the room with great success driving by Tact 2150 amplifier using no more than 5 watts per channel. The specs is available at the Precision Devices website as well as plenty information on the Web.
I have only 1 box for 1 driver, another box had been misplaced. I would prefer local pickup as each driver weight around 70 lb.
I set condition to 7/10 only because of age - around 5 years. These drivers are in perfect working order and can handle extreme power.
Too large for my place. Can be used in sealed box of 4 cubic feet, bass reflex of 12 cubic feet and also in the Open Baffle as it was done by some audiophile in Europe with big horns systems.
Thanks for looking.