Dual subs will smooth out the frequency response in the room and increase headroom. The quality of the sub will determine the SQ for the most part. The integration will also affect the SQ, so setup is very important. In short, 2 subs are better than one when setup correctly and the Bash or Sledge amp is not a major factor unless high levels of spl are generated leading to more distortion.
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, Samsung BDP F 7500, Asus/My Book Live HPC 4 TB
Yaquin VK 2100 amp, McIntosh XR 5 speakers, Samsung BDP F 7500