I'm running four budget subwoofers in my rather small HT.
I have a Onkyo TX-NR3009 receiver with Audyssey MultEQ XT32 with Sub EQ HT.
I leveled each sub individually to 80dB, prior to running the full eight position Audyssey calibration.
My subwoofers include two ported and two sealed models.
Audyssey did a magnificent job of blending all four subwoofers seamlessly.
The bass is clean, tight, powerful and oh so smooth over all the seating positions.
Benq W1080ST DLP 3D Projector
Sony BDP-S5200 3D Blu-ray Player
Denon X4000 Receiver
Pioneer SP-FS52 Tower Speaker x 4
Pioneer SP-C22 Center Channel Speaker
PSA XS15se Subwoofer x 2