Subs?

Two subs are always recommended over just one.  You can mate subs with any speaker regardless of size.  The size of the subs should be in accordance with your room size.  A couple of high quality 12" subs would be ideal for your application.  Happy listening.     

A.  Whether you decide on one subwoofer or three its imperative they be positioned within your rooms standing wave bass modes. This is a relatively simple way to locate and mark your rooms modes and is only done once.

https://www.audioholics.com/home-theater-connection/crawling-for-bass-subwoofer-placement

Using any other locations and you'll find yourself as dissatisfied as all those you read about griping, struggling or who've given up. DSP is not a panacea for out of position subwoofers.

B.  Using a distributed subwoofer array or Swarm essentially eliminates the rooms modes and frees you from preforming the crawl. It also provides a unique and extremely encompassing low frequency presentation.

C.  The last option is to use a -6dB sub-bass speaker (some also call themselves subwoofers). Regardless of how many are used or how large they are their frequency response begins a steep roll of in the mid 30Hz region. Since they simply do not excite the rooms standing waves they can be located most anywhere, at the expense of losing the true subtle musicality that a properly positioned and adjusted -3dB subwoofers provides.

Personal observations: More often than not speaker manufactures tend to design afterthought or needlessly over built premium priced subwoofers. They offer uselessly dated setup diagrams and/or complicated, or dealer adjustable third party signal processing.

High or speaker level connectivity was originally developed for customers whose receivers were without preamp outputs. If it were actually better everybody from Pro audio, sound reinforcement, and the majority of home subwoofer using it. Running the source signal through at least two more sets of inputs and outputs?

If the company provides you a list of speakers that their products are "matched" to, your in way over your head. Put simply, one adjustes subwoofers to any speakers not matching a particular speaker.

My current two subs have taken the place of four. It took this idiot less than an hour to crawl, position and run the Auto EQ which is 95% of the setup. The other 5% is making manual personal taste adjustments. The  newer 12" sounds every bit as potent as the 18" it replaced. Unfortunately the company was sold in 2019 so I'm reluctant to make a recommendation.  

Pull off your pants and jump in, you'll never go back.

I say more the merrier. They add depth to my system. I started with one KEF R400B, Each sub has 2x10" speakers and a 200 watt internal amp. That was good, so I added a second of the same. I cross over at 40 hz. They are very adjustable, and you really don't know they're on, until you turn them off. My main speakers are Sonus Faber Cremona Ms. I'm contemplating adding a third. 

2 subs are better than one in most rooms and probably in your room based on your description.  I started with one SVS SB1000 Pro.  It sounded good.  I got a second one and it sounds even better.  

1st you have to decide your on your priorities… Do you want Music Bass or Movie Bass ??? The Experience is totally different with each… So what is your main listening going to be ?

I went from (1) SVS Front Firing 12”er to (2) REL 8” Down Firing T-5X’s. The SVS was ok for movies but for music it just didn’t cut it. Was it only because I had one ? I couldn’t tell ya but the 2 RELS are much more musical and dialed in so easily. I found being they had smaller drivers the response was much faster and worked better in my smaller room of 12’ x 12’. Being a smaller driver they have a lot less air to move. I have them dialed in at 45.25hz and volume level as one noted before at “ A Whisper” and couldn’t be happier. No Localization !!!! The secret to a sub’s successful integration into your set up is to not know you’ve got subs, but would seriously miss them if they weren’t there. Wanna drive yourself nuts? .. Nothing worse than hearing bass coming from a specific location in your listening room and having multiple subs certainly balances things more evenly.

Being my PreAmp has HT Bypass I do have the RELs attached to my DENON 3700 AVR as well through the AVR’s LFE output to the REL’s “SEPARATE” LFE input.

NOTE: REL’s allow you to have High QUALITY inputs from BOTH

(1) directly from your Audio System’s Amps through the speaker outs sharing the output feed that goes to you speakers and

(2) also has SEPERATE LFE inputs for an LFE signal from an AVR with the ability to Automatically switch incoming signals according to the input signal being feed to it.

The bass response for movies is great but music is my key objective and I couldn’t be happier than I am with the (2) RELs