Subs?

IMO, too much money is spent on main speakers that try to accomplish too much with respect to quality and quantity of bass.  Too often the result is boomy, ill-defined bass unless you spend a lot of money to get it right.  Your 804s are well-suited to provide enjoyable, balanced sound from around 40Hz up.  Take the win.  Adding subs will enhance your experience and improve on what you hear at the higher frequencies.

IME, two subs are way better than one and even more can be better yet.  Google "Swarm Subwoofer System" and "AudioKinesis" to read about the potential benefits of using an array of subs.  Not everyone can either afford an array of subs or get away with having 3 or 4 subs in their room.  However, if you can swing two subs I suspect you would be happier with the result compared to only using one.  I will let others weigh in on which brands/models of subs to consider. 

My Aerial SW-12 subs are plenty impactful, musical, and adjustable for me so I never needed to try others.  I have had good luck with them in the front of the room, set up for stereo, using line-level connections, and dialed in with a light touch. I have tried a passive high-pass filter to relieve the amplifiers of the chore of driving the lowest bass, and to relieve my main speakers from having to reproduce the lowest bass.  In my case it was a wash and not a noticable improvement but for others using a high pass filter has been beneficial.  Good luck.

A sub can be just as important as a tweeter and choice of sub will depend on the type of speaker you have.

Take a look at (or listen to) to the DB series B&W subs. For just music I'd take one of those over a JL all day long.

+1 on two subs, so size might be predicated on space available and cost.  That being said, ideally I would take two 12" subs over two 10" subs.  I am a REL fan, based on experience, sound  and reliability.  There are many reports here of JL reliability issues.  I have heard the B/W subs and would choose REL over them.  I also replace two Paradigm subs with REL and again they sounded much better.  I am sure a swarm sounds great but even for my excessiveness not practical.  IMHO, get the best REL subs you can afford.  You may hear that REL subs do not have some of the sophistication in adjustment (variable phase and remote control), but I have absolutely no issues with dialing them in.  They add air pressure, bass presence and depth, and augment the soundstage if placed and adjusted properly.

Matching brands is not the leading concern either.

Definitely at least two subs, and three or four is even better if you can swing it.  I’d go for 15” subs but no smaller than 12” because they’ll go lower and work more effortlessly with lower distortion than smaller subs.  I think Rhytmik subs offer the best performance/$ although they don’t as yet offer software to help dial them in, which is not necessary but can be very helpful.  Arendal subs have also gotten good reviews and seem to offer high value, and I’d also consider Paradigm and Perlisten.  B&W is overpriced relative to alternatives IMO, and JL Audio has had significant reliability issues that may be more prevalent in their lower-level subs but worth checking before going that route.  I want my subs to hit an honest 20Hz because there’s important information down there that improves sense of space/soundstage, and to get there with REL you gotta spend big bucks relative to these other excellent alternatives.  If it’s me I’d go with a Swarm from Audiokinesis, but if that’s not possible I’d go with a pair of 15” Rythmiks and take the thousands you’ll save and upgrade other equipment or go on a nice vacation.  Hope this helps, and best of luck.