Get as nice a sub as you can afford and later get a second sub. Use the Polk sub in a nearfield setup for now and sell it down the road. This will give you a little extra cash for getting identical subs or 2 subs very closely matched in their performance. A good sub will make a bigger impact than an amplifier at this point. The thing of using identical subs is over hyped imo.
Klipsch RF 7 based HT 7.4, Pioneer SC 35, Acurus 200 Five, Dayton 18 Ultimxa Dual Sub Cab(2), Dayton 18 Ultimax Large Vented Sub Cab (2), on Berhinger I Nuke DPS amps, Aura Bass shakers, Samsung BDP F 7500, Asus/My Book Live HPC 4 TB
Yaquin VK 2100 amp, McIntosh XR 5 speakers, Samsung BDP F 7500