No reason at all not to. Just keep the drivers the same distance from your listening position as the mains to maintain time coherency. Here's a webpage that has info on Roy Allison's, "Allison Effect" papers, and some programs to aid in the proper design of subs using Thiele/Small parameters, transmission lines(my favorite), etc.: (http://www.termpro.com/programs/archive.html)
dumb sub question
just sold my last sub (velodyne spl 1200) and want to replace it. my question is that instead of buying the latest fathom or whatever, can i not build a killer sub for way less money? i've been snooping around sites for drivers, diy boxes etc., and have my eye on an eton 12" woofer. i have a paradigm x-30 crossover, access to all the mdf of any thickness i want(for free), and would buy a seperate power amp to drive it (nad carver adcom etc). i do not have a tv, so it's for 2 channel only. i listen to mainly blues(usually turn the sub off)and heavy,heavy metal(that's where i want to feel like i'm getting kicked in the stomache!). i figure one 12" or two 10's (sealed box), and think i only need to go as low as a true 40hz. is there a good reason for me not to go this route?