Subwoofer Suggestions ?

SET-UP: Samsung HL67A750, GS20, GSLCR, GSFX, Servo-15, Oppo-95, Rotel 1570/1575, Belkin PC, Belkin UPS

my room is 17/22/9
should i get 2 sub 12`s or 2 sub 15`s ?

does anyone know how the DD+ compare to the sub12/15 ?
Best to have two, twelve inch units. Check out Jim Smith's 'Better Sound' for details.
I'd get two 15's if you want the best bass. Big difference between a 12 and a 15 all else being equal.
I'd think that - in your room - two well designed (important!) 12" subwoofers should do the trick very nicely. My pair of 12" Rythmik f-12s (each w/370 watt amp) in a similar sized room are set at very low levels so that each can produce the 75db reference tone specified for Audyssey set-up. Caveat: It's hard to translate that to specific conclusions re: clean output capabilities at low frequency (the main reason you'd move to 15" drivers), but:

My guess is that there's a fair bit of untapped driver excursion/amp power (i.e. clean output potential) left in this set-up before it gets near it's limits. It sounds utterly unstressed at HIGH volume. I suspect that you'd have a similar experience.

Good Luck.

The Martin Logan Depth is the best sub I've ever used. Don't own one now, but have fond memories of it.