Welcome to AVS. I'm going down this learning path as well. I have a second subwoofer in shipment right now that is a good bit more capable then my current one. Look forward to hearing some recommendations here. This will also be my first multi sub attempt so I'll be following.
I've found a ton of reading and approaches on this, perhaps tomorrow I'll dig all the links back up and throw some up here for you but you will find a ton of information.
Of course, there are multiple schools of thought here. One of the approaches places the more capable subwoofer up front in a sub sweet spot (see sub crawl) then the less capable sub (s) are used to tame modes placed along side walls or rear. They are recommending two less capable mode tamers only operating on certain freqs and not trying to match levels for those as they are just filling in the holes. This is the Geddes approach.
Another approach is a more textbook approach with perfect rooms, matching subs, geometrical placements... things like 4 equal subs along the middle of each wall. Works out great on paper from what I've read but the real world just isn't that tidy. This is the 'Welti / Devantier' approach
It should be an adventure finding the best setup. I do plan on using REW for measurements and a minidsp to help me out with PEQ. Highly recommend REW if you have not done so yet. My approach will be to try something, measure, try something else, measure... :-)
OKOK I'll dig up one link http://blog.acousticfrontiers.com/whats-new/2010/10/29/using-multiple-subwoofers-to-improve-bass-the-welti-devantie.htmlEdited by dstew100 - 6/7/12 at 2:11am