Whether or not you want two subs depends on what you are looking for and where you are putting it - Ritz has a pair of LFM-1's and usually leaves one turned off, I have only one and am quite content with it, but if you had a very large room to fill at high SPL's you might prefer two.

The SMS-1 forum here has been getting a good bit of traffic lately, with a variety of people using a variety of subs. My room is better behaved than I could expect for as little acoustic design as I put into it (I credit a mix of luck and convenient but unrelated decorating choices), so my peaks and dips were pretty mild and I still got some detectable benefit from the LFM-1 - it can do tons more for you than the LFM-1 alone allows (or most any other sub, for that matter). A room with more significant problems but that you couldn't readily address with acoustical treatment would likely reap significant benefits from an SMS-1. You may be able to use a mix of test tones and SPL meter to make a guess at how much problem you have if you aren't sure you want to factor an SMS-1 into your overall budget.