Small or large sub for music

Large subs go deeper, better and louder.

HOWEVER! Larger subs are harder to match to the room. The lower they go, the more room modes you are likely to find. 

With proper room treatment, and good EQ, a good 15" sub will do music or movies absolutely superbly with any speaker. 

Without those two .... a small sub is far less dangerous and more likely to work for you.

Erik,

I use the HGS-10s and 15s with SMS-1 bass managers that provide acoustic room correction.  I noticed when I played the Saint-Saens 3rd symphony that the pedal notes are not as well realized by the HGS-10s as they are by the KEF Reference 107/2s.  I think the 10s extend to just below 30 Hz whereas the 107/2s and 15s extend to below 20 Hz, so pedal notes have that more felt than heard vibration I sense at live recitals.

db

Just get a larger room, sorry thats not an answer...big woofer more full deep bass, small woofer better tunefulness most the time, big woofers are better for some , small better for others, depends on what sub what room, what you like... thanks
Matt