Dual Subwoofers?

2 subs much better than one.  A couple SVS SB1000 Pro subs will get you down to an honest 20Hz with minimal size at less than $1000 and absolutely transform your listening experience.  

@mikeydee , Above mentioned or the SB2000 Pro would be the most affordable solutions if you have never heard dual subs set up correctly before...and looking to get your feet wet.

Price goes up from there...

Here are some very good subs ... 

Rythmik F12 (12 inch), F12G (12 inch), G22 (dual opposed 12 inch) 

KEF KC62 (dual opposed 6.5 inch , KEF KC92 (dual opposed 9 inch)

Elac Varro DS-1000bg (dual opposed 10 inch), DS-1200bg (dual opposed 12 inch)

I've been using dual stereo subs for years.  Very happy with that decision.

+1 et al.

I had one SVS SB300 and it was very nice. I then got a second and everything got much, much better. Stereo separation and imaging improved.

I'm considering a pair of martin logan subs, heard them and like them. Lotsa setup/control features.