More Power or use subwoofer to boost bass for music

i don't remember what the dealer use.  It was actually in Magnolia shop of Best Buy when I heard them for the first time.  I think they might just be using a Rotel integrated.

Any particular reason you went with the A31 vs the A21/A21+ or even JC5? While a swarm might be one of the options for big low end, often they are impractical space/position wise and still require proper setup to get right. So reality is, any sub setup will require proper placement and dialing in. You might be better off to try the above first, room treatment, speaker placement etc. These are all things that need to be done anyway if a sub/subs are part of the picture. Also, cabling can make a big difference as well, power particularly as well as the rest. 

Reason for A31 is because I only run a 3 channel (L,R,C) home theater.   I don't have room for surround speakers.   I have the Marantiz 8802A preprocessor for both movies and music (via Pure Direct mode).    I'm also looking for a stereo pre-amp right now.  I'm considering Parasound JC-2 or Mcintosh C49.  I actually had another post in this forum about that.

Sometimes amps are the problem, if you can get a loaner, go for it. If that doesn't help you dramatically, look to the room.

I worked at Magnolia for a bit, and their standard setup had the 804s positioned very close to the back wall so maybe not surprising you remember hearing more punch when you were there (I used to pull them 4’ out into the room where they sounded much better albeit with less bass emphasis). Regardless, 804s have never had prodigious bass output and could certainly benefit from at least one good sub. As @noble100 mentioned, the improvements extend beyond bass to expanding the soundstage and overall image palpability. I’ll never forget the first time I heard a good system with properly dialed in subs. Of course there was the expected bass depth and impact, but when he disengaged the subs the entire soundstage collapsed and the sound overall went limp. You can certainly achieve better sound with room treatments and EQ, but there’s really no substitute for what good and well-integrated subs can do. I like the suggestion of a pair of SVS SB1000s that run $950 and offer a generous in-home trial period, but I seriously doubt you’d be returning them.

One last thought — if your speakers are new and don’t have at least 100 hours of quality playtime on them you may not yet be hearing all that the 804s have to offer.  In any event, best of luck.