Best Subwoofer To Use With A Two Way Speaker System

Ive had very good luck with REL.  A pair of Subs has worked far better than one.

I have a pair of REL Strata IIIS with Decapos. Agree with Chlv0ter, adding a 2nd sub was as significant as adding the 1st one and not just for bass performance but overall smoothness, dynamics and clarity, just more musically convincing. These RELS integrate so seamlessly, they never impose as I have heard with other sub systems.

If I had the room to add a 2nd REL and could locate one in my area, I'd definitely try it.   I believe in what the commenters say above - what one does well, two likely do better.   It's not happening in my 11x10 room, but if I had the space for it I'd love to hear what a 2nd REL would do for the sound.

Hey, Perazz...
I recently purchased the JL Audio e112 to add the extra low end to my B&W 804's.  My decision to purchase the sub was due to a review (yes, I know it's not the best way to purchase a piece of audio equipment).  But I did have a small fear the sub wouldn't blend as well as I wanted it to.  Well, my small fear diminished once I got the sub dialed in to the system.  the extension is almost heaven.
But I knew there was more that could be done.  I looked into how to improve the bass even more - and that's when I researched a sub acoustic platform which essentially eliminates any vibrations from the sub to be transmitted into the floor (and thus into the walls).  This little 70 dollar addition transformed the bass even more...was more fluid and blended even better than before!  That much closer to heaven...  I "cannot" stress how much acoustical treatment will transform the sound for you.  Good luck in finding your audio nirvana

Another +vote for REL.  Happy Listening!