With only those choices and for "Free", the DD-15.
If I had to actually pay for the subwoofer, then my answer would be this:
I would
personally go with the seperate SMS-1 EQ ($599) and either the Earthquake MKV-15 ($1400 from authorized dealers) or the Maestro ($2100) from ACI and save some money. This is just my opinion as I have heard the DD series, ACI subs and the MKV-15.
Above 25Hz the MKV-15, imho is very close to being as good for music as the DD series (and the ACI Force which I have owned) which I've demo'd before in my room. Someone over on AVS actually liked the ML Descent better than the DD-18 by just a little for music. But not sure how good it does in the SPL and/or the HT department.
I do know that the Maestro (and MKV-15 for that matter) both have more SPL than the DD-15 for HT down low. If you check the charts over at AVTalk on the Servo-15
v1, you will notice the DD-15 does better, however the
v2 has like 3-4 times the output of the
v1 above 35Hz as stated by Ed Mullin and Illka. And according to a guy who tested both the Servo
v2 and the Maestro side by side in the same room measured them as about equal in the SPL department (and preferred the Maestro for music). And just to compare to the DD-15 for example at 20Hz to the Maestro. The DD-15 max. (GP) SPL is 92dB at 20Hz, and the corrected GP measurements of the Maestro are a about 100dB at 20Hz.
Anyway, there are always choices and they should be based on more than just numbers. Max SPL at any given freq. and at what level of THD is one thing, but there are a lot of other variables that one should consider. How low do you really want or need a sub to go? How much does one use it for music and movies? How big is your room, how loud do you like it, and do you have size restrictions of the actual subwoofer itself? How much money are you willing to spend?, etc.... BUT, in the end it all should come down to personal choice. I have no doubt that if say the DD-15, the ML, the MKV-15, and the Maestro were compared directly to eachother in listening tests in the same room without looking at graphs and without knowing which one is playing, some people would have definate preferences to any of the four. Some I'm sure would be happy with any of them.
If someone is in the market for one of these quality subs and have the money and the time, try them all out. Hopefully you can do so either by 30-day trial (in the case of the Maestro, pay only return shipping if you don't like it) or by a dealer loan. Or if you really want to know, buy them out right and be prepared to sell the ones you don't like after you have figured out which is best for you to
your ears in your room.
I would whole-heartedly recommend an EQ like a BFD or SMS-1 if you don't decide on something like the DD series subs though.
Btw. throw in the likes of the higher end SVS and Hsu models too. If one is planning on paying $4000 for a sub, they can afford a few return shipping charges (though with SVS and Hsu you will have to eat shipping both ways).
Anyway, that's my take on the it. Just another opinion of the masses.
Steve