I'm curious why people are recommending a subwoofer upgrade when the poster is looking to improve music listening? How much content is there below 80Hz in your average song? I'm talking rock'n'roll, not pipe organ.
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I know Audioholics recommends 2 subs for a good size home theatre, so a second Paradigm sub might end up being better than replacing the existing one for home theatre use, or replace the Paradigm with a couple others?
For music listening though, I would side with Pogre in that a speaker upgrade would have the biggest impact. I auditioned the Paradigms when I bought my home theatre setup and personally I would have to spend a lot more money to get to the next level. I brought a wide variety of music to the shop and I really like their sound. My personal recommendation is take your time. Think about what it is that you want to achieve. Are you looking for a wider or deeper sound stage? More life-like vocals? Something lacking in the bottom end or is it the mids or highs that need refining? With that goal in mind, visit some hi-fi shops and try listening to a good variety of speakers. If your room allows for it, check out electrostatics or reflecting speakers like Ohm Walsh (if in your area) as they offer a very different experience. If you find that nothing significantly improves over the Paradigms, then you can look at tweaking what you have, whether it be amplification, extra subs, or room treatments. And if you're streaming music from a computer, maybe look into Roon and Roon enabled devices. I know, for me, half the fun is in the research.