Which subs are best for home theater?

I now have a double 12" self-powered sub that I'm using in the front channel for home theater. I am thinking of adding a rear-channel subwoofer. My dealer has suggested using a single 15" down firing sub in both front and rear channels. Would a 15" in the rear be overkill? Should I use a 15" for the front? And maybe use the double 12" sub for the rear channels?

Another question would be is a double 12" sub better on audio than the single 15" sub?

Thank you in advance for your suggestions.

Wow, what size room do you have that you need two subs with 15" drivers? I do agree about the downward firing subs, but I think you should look for a smaller driver to retain speed. I think this calls for a REL Stentor II, or if you're feeling really frisky a REL Studio II. Don't hurt yourself now...
I use 2 ASW-800's in the front chanels as subs and one in center to make a full range center chanel. I also use two BAG END infra sub 18's in the rear with life like results and I never find it overpowering. My dedicated theater/listening room is 13' X 26'. I use the ASW-800's (right/left) for 2 chanel audio also and I have had many compliments from audiophiles on the sound. Good Luck and enjoy!
...meatball supreme subs are the best that I know.
It is a no brainer, Velodyne 18's, either the HGS-18, FRS-18, or F-1800II.

You will not find a better sub for home theater, and if you literally want to move pictures, run a pair up front!
Thank's everyone for your input , great feedback! mr.upgrade
I had an 18 inch Velodyne and ended up with a Rel. They each had their strong points.

The Velodyne will shake pictures off your walls and it moved so much air that I swore I could feel a breeze from it sometimes. The down fall with the Velodyne was that no matter how much I tried to intergrate it, like all foward firing subs, it was always very directional.

The Rel was the exact opposite. They dont move a lot of air (comparativly) but not only are they seemless but they produce a lot of fine detail. Intitially I pefered the Velodyne but after watching a few movies I found the Rel much more involving and the extra detail brought a level of realism that far outweighed the volume and air movement of the velodyne.

If you go far enough up the Rel line you will get the best of both worlds but they do get very expensive. I recommend buying a Rel and down the road when you want to upgrade add a second one and run stereo subs. The overwelming benefits of stereo subs is suprising and the extra bass is always welcome. If you go with a foward firing sub I highly recommend putting it on a 6 inch stand which will take out some of the boomyness and help intergrate it.
Take a look at the Paradigm Servo 15. Im delighted with mine.
I definately think the Talon Roc gives the best of both worlds for home theatre and music. It has the speed of the Velodyne and the integration of the Rel. I really like it with my Martin Logan set-up.
I hadn't breakfast today and I'm waiting to get some Pizza Hut meatball supreme sub now.
i have tryed the servo 15 and also definiteve sub
but as for now i have velodyne hgs 15" witch is the best one ever had.
you can fill the air moving around and the exsplotion are real to life.
try the velodyne hgs-15/18 you cant go worng with them.
Marakanetz, does that accomplish the job of getting the air moving?

Do you find it difficult to integrate with the main course? Which direction does the main pressure come from, front or downward?

Hope these questions are not too technical.
albert: good questions. and i will add my own: are you, marakanetz, as i assume, also retromingent? -cfb
Albertporter: it's all depends on how big the sub is...
I would have to throw my vote for an Earthquake Supernova MKIV-15. It uses a 15" woofer and a 15" passive radiator, and can pump out extremely high levels of sound well below 20 Hz. I can sell these subwoofers for less than anyone on the net, so if you are interested, please let me know.