Subs?

Two subs are always better than one. They extend the soundstage. 

This may be true, but for me it is subordinate to them helping with room modes. Two is good, three is great, four is...marginally better.

Helpful:
https://blog.teufelaudio.com/what-are-room-modes/
https://forum.audiogon.com/discussions/subwoofers-and-phase-question-for-you-sub-experts
 

I added 4 subs and it was the best bang for buck ever.

Swarm, DBA, Audio Kinesis, and James Romeyn…

https://jamesromeyn.com/speakers/subwoofers/general-info-subwoofers/

I made the change about 4 years ago with help from JR Bosclaire (Wally Tools). And when I say help, I mean he did all the work and I held the mic all over the room.

The thing that blows me away to this day, aside from the dynamic sonic improvement, is how little it cost me relative to my system and to mainstream audio products.

I totally get the feeling that four subs is a lot. I debated that as well. I also felt my family would lose their shi$ if I added more “boxes” to our family room (we have an open floor plan). An advantage of a DBA is the subs are asymmetrically placed around the room and the size of Jame’s implementation are not really that big (especially compared to what’s out there commercially). There was definitely resistance at first, but I got lucky. My wife and kids recognized this hobby is a passion of mine and after a while we all sort of just forgot about them. Just remember everything takes time.

Anyhow that’s a little more context for you to think about.

So, one is better than none. Two is better than one. And so on…

I hope you will let us know what you decide and report back.

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.