Subwoofer recommendation and wisdom

Welcome to the deep end, you may never go back.

You can definitely augment your extra low frequency presentation using just one -3dB rated subwoofer with one caveat. The subwoofer must be positioned within one of your rooms standing wave bass modes insuring proper room integration and allowing its full potential for speaker integration.

Since identically dimensioned rooms can have different mode locations it's imperative the very first step is to preform the subwoofer crawl to identify your rooms multiple mode locations. This will identify your rooms most ergonomically convenient subwoofer positions. Manufacture position diagrams are worthless and mathematically calculated positions do not include a rooms furnishings and treatments.

Using a repeatable low frequency cadence between 20-35Hz, found on line will make identifying your rooms standing wave much easier than a musical passage. Custom length interconnects from Blue Jeans or Monoprice Cable are economically sufficient for subwoofers. 

Once positioned proceed by using the subs available adjustments to integrate it with your speakers to your personal taste.

-6dB rated sub-bass woofers which are usually sold as 'subwoofers' are typically so rolled off as to not excite a rooms standing waves enough to identify the rooms modes. They can be located most anywhere at the expense of the delicate extra low frequency (not sledge hammer) content present on much of contemporary recordings. 

A quality subwoofer investment can be used with many future speaker replacements. Enjoy.

Was in this situation, added one sub. It was great, but still lacking. Added another cheap sub and it was much better, but still lacking. 

Then purchased 2 new modern subs with DSP, this was a total game changer, never knew bass could be this powerful, full, have so much texture. 

Weirdly, it will make your main speakers sound better, not sure if it's due to turning down the bass on the mains, the crossover or what. After dialing in the sub, sound stage was deeper, everything was cleaner, fuller, more detail. My main amp is working far less, and I'm getting far more out of my system.

One thing of note, usually if you are getting 2 subs, you can go down a size/level and still get more out of 2 smaller subs over 1 bigger. Where you would spend $1500 on one sub, 2 $700 subs will give you more bass. Currently using 2 10in subs, gain is set at 26 out of 100, no tone controls or loudness being used. At half volume on my system, pictures can fall off the wall. It very much pressurizes the room.

I'm your Huckleberry. Get a size/budget appropriate Rel. Put it in your seating position, and do the sub-crawl. 

@mswale 

One thing of note, usually if you are getting 2 subs, you can go down a size/level and still get more out of 2 smaller subs over 1 bigger. Where you would spend $1500 on one sub, 2 $700 subs will give you more bass.

Yup. I'm not sure about 'more' but wider coverage with more consistent SPL vs 1 bigger sub having a hot spot in the center and cool zones near the edge and varying SPL in between. Overall better quality though not necessarily higher max SPL. Also you have less risk of room modes from too much bass in a particular area (not room).

I don't agree with any sub crawl suggestions. It's hard to localize bass below 40Hz. A mic does a better job.