Do I need a subwoofer?

In addition to adding bass, properly set up subs will also greatly improve imaging and expand the soundstage.  I’d recommend trying a pair of SVS SB1000 Pro subs you can get for under $1000 (they were running a sale recently but not sure if it’s still going) that are relatively small but will still get you down to an honest 20Hz and also offer integration software to help you get them dialed in properly.  And yes, you want two, not one sub — huge improvement.  SVS offers a generous and truly risk-free, in-home trial so if they don’t transform your system you can simply return them but highly doubt you will.  Hope this helps, and best of luck. 

Those 15" woofers ought to be giving you plenty without potentially adding mud.

Perhaps, if space allows, go for the non-directional multi-sub ’swarm’ solution. Then you add a room filling BASE of bass, which transcends up to directional bass from those 15" woofer’s emanation of low primary and their overtones which reinforce directional cues.

You might think: add the sub(s) ONLY for Video. My small HT, not what you are dealing with, I have a single self-powered sub, I keep the crossover set low, essentially just add something for Jurassic Park Dinosaur Stomp, not aware of it unless it is off.

On for music, no room or budget for swarm, a stereo pair of self-powered subs, no ports (or front port only), located adjacent to the mains.

All - this is super helpful. I am enjoying being back in the research game after years doing nothing to my system (looking to replace my preamp, too - that's a different story). The SVS line looks like a good place to start.

This comes up so often I wrote a post about it:

@erik_squires That was a nicely written piece. Very informative. Room treatments has been something I have lacked in.