I use a pair of Rythmik 12" subs in my music only system. Measured response in my room is flat to 30hz(technically, it's down app 2 or 3db)without EQ. I use a Velodyne SMS-1 sub controller to smooth a couple of room nodes and extend response to 25hz (-0db). This covers about 99.999% of all musical signal with impressive on-axis accuracy. These subs blend smoothly with my Ohm 100 or Maqgie SMG mains (I use the SMS-1 to optimize the x-over function) for a neutral balance, wide bandwidth system at a reasonable cost. The Rythmiks @ app $1500/PR. + sms-1 @ $450 + Ohm or Maggie $600-$1700/pr. make for -IMHO- a speaker system with an unusual combo of strengths at the $2500 to $3500 price point.
I'd note that the Rythmiks replaced a pair of smaller, similarly priced Velodyne SPLR 8 subs. The Velodynes were very good, punchy sounding subs, but the Rythmiks provide a "drier", more relaxed, more neutral sounding bottom end - to my ear. Not surprisingly, they also go much deeper/louder without the need for EQ.
There appears to be some other fine value subs out there (the 12" SVS models come to mind) at/near the same price point. I know they measure well, (test results suggest that the SVS might be even better than the Rythmiks for HT use) but I haven't actually heard them. OTOH, for music only, I can certainly recommend the Rythmiks very, very highly.
Good Luck
Marty