Most accurate sub under 1,500 used

Get a DD-15.

JL F113 or SVS PB ultra 13. Take your pick. these are both very good (low distortion) above 40Hz. The SVS will be more musical with at least two of the ports plugged (lower group delay). The JL F113 is musical from the get go.

So far, I'd say a DD15 too.... or a DD18 if you can swing it.... but your room should determine which, as much as your wallet.

How much will I gain over the DD12 with the DD15. The 15 will go a bit lower to 15Hz and the 12 is 17HZ. The DD18 is out of my price range.

How much will I gain over the DD12 with the DD15

A lot. If you went with a DD12 you might want two while a DD 15 or DD 18 would probably be enough.

Shadorne, are you talking about more low frequency extension or sheer output?

I have a DD-12 in 15 x 20 room with 10 ft ceiling. The volume is set to ~20 comfortably, and I have never maxed out on the volume or bottomed the woofer. You should still get the largest that you could afford, but if it comes to DD-12, I think it could work just fine in your room.

Shadorne, are you talking about more low frequency extension or sheer output?

Output at low distortion. Bigger is better. FWIW - the SVS PB13 is exceptional at low distortion at ultra low frequencies due to the ported design (great for movies), however, I think you'll find you want to plug those ports for music - and the DD-15 will give your more musical bass drum slam output between 40 and 80 Hz...