Subwoofers:HT and music:DIY

I am interested in members' opinions on this woofer system.
I will use these on my HT set-up as the LFE/.1 subs,to free up my DIYs for music.
I will be using a 9cuft cabinet.Choice of woofers are Dayton Reference 12"-4 (315) pair or single 15" Dayton Reference 4ohm or Titanic III 15".The bottom end is between 19-23hz.These cabinets will be run as a mono'd stereo pair (one each side of room).
Big question :2-12"s or single 15".I'm leaning toward the "two",as I can run them parallel to take maximum advantage of the 240W/4 ohm sub amp.
PLease comment directly upon choice of drivers and exclude all references to "other" drivers/subwoofer systems.Not looking for "sales pitches or going off thread".
Thank you for your opinions-in advance.Tom
Thought I would follow-up my own thread.
I had allocated about $650 for two subs,to use as the .1/LFE units in my HT.I decided to go "used",to hopefully improve upon this aspect and free-up my DIYs that were in service.My decision:I bought the NHT Evolution U-1s/A-1s and X-1 crossover.These should be here near the 13th of February.I am hopeing that these subs can move some air and deliver the slam of the previous pair of 15"ers.I'm sure that it will take me some time to blend them in,but it should also allow more room "behind" the system and I won't need to turn-on the plate/amps again.