Subs?

For dimensional reasons I went with one SVS 3000 Micro (2 at 8") and it really made a difference, notwithstanding the 15" woofers in my K-horns (which only go to 33 Hz).

The second, identical sub really sealed the deal on improved SQ; it also has me wishing I'd been able to accommodate larger subs.

+1 for multiple subs. If your mains go very low, like your 804 D3's, you're stuck with trying to balance two issues: imaging (driven by speaker placement) and room modes (dictated by speaker placement). You would be incredibly lucky if your solution to these two problems happened at the exact same speaker placement!

By separating the two, you get to put your mains in the ideal place for imaging, but get to move the low frequency sources (the subs) wherever is ideal to minimize problems with room modes. Conquering modes is where "the more, the better" mantra with subs comes in.

To really get this separation, you need to high-pass the main speakers, and low-pass the subs, so the sound level and phase are (mostly) matched. Using an active crossover and/or DSP are ways to achieve this.

I think you are going to struggle to find two 10 inch subs that will get down to 20Hz effortlessly and provide enough air movement to pressurize the room.

I'd go with whatever brand/models that you can get two of that can do 20Hz easily.

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.