Dual subs and room size

Hi. I've been considering dual subs with speaker input to pair with my Martin Logan 35xt's and not sure what I should be considering in relation to room size. My room is 17' X 10' with the equip along the long wall. I've been looking at SVS sb1000s and HSU vtf 1/3. The SVS are 12" sealed vs 10" ported hybrid for the HSU. It was suggested that I go with no more than 10".  They will support 100% music. The speakers are pretty much an 9' equidistant triangle set up and would be looking to set the subs on the outer side of the standmounts. Any thoughts or advice would be greatly appreciated. Thanks, Dave

While we're on this subject, I have a question. Did I read someplace that newer stereo releases are being mix/mastered for separation down to 20Hz (and below)? Would this be a good reason to go for two subs?
There are several industry experts which will only recommend 2 subs in a 2 channel system.  George Vandersteen and Jim Smith to name a couple.  To the point they state, for a 2 channel system,  they would not add a sub to the system until they had 2.  In my own experience,  I owned a single JL Audio F212, and I was never able to get it to blend right with my speakers,  and I felt it took away from the imaging of the main speakers.  It wasnt,  due to lack of trying, I worked for weeks on sub placement and in different rooms with different speakers,  B&W 802D2,  Revel Salon 2,  Aerial Acoustics 20T V2.  Rooms all treated.  After researching,  ultimately bought a second F212 and was blow away by the improvement.   In fact,  I didn't tell my wife I bought the second sub and was playing the system, she said,  "wow,  that is sounding really good".  What did you change?"
Again I want to thank you for your advice and guidance, I have been following your leads. Last night I spent some time moving and adjusting my current ML orig model Dynamo sub. Wow what difference. Will continue to lock it and conduct measurements. Will be ordering one sub with a thought to two after I lock that one in. Very interesting process. Dave