Nathan from Funk Audio mentioned something like this to me once: Your subwoofer sound quality won't reach it's full potential without some kind of good EQ.
I didn't want to believe him at the time because I always thought "Equalization" was a dirty word.
But after comparing Audyssey Dynamic EQ On vs Off, my interest was 100% piqued.
At first I thought maybe Dynamic EQ just pumped up the subwoofer level and made it "HOT". So I pumped up the subwoofer level to +6dB and +9dB HOT. But the sound was nothing like with Dynamic EQ On.
So I concluded that it was the Equalization of the mid-bass by Audyssey Dynamic EQ.
Too bad you don't have a Denon or Marantz with Audyssey Dynamic EQ to try out.
I don't know if your Pioneer has some kind of Dynamic EQ for the bass.
Adding another sub may increase the subwoofer bass level and may smooth out some spots in the room.
But unless you have some kind of good bass EQ, Your subwoofer sound quality won't reach it's full potential without some kind of good bass EQ.