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?
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)
I forgot: You mentioned 40hz as your goal. You'd be amazed at how much ambiance info is in that next octave(20 to 40hz). If you were able to find a used Hafler TransNova 9505, and built a pair of 10" TLs: You'd be in bottom end heaven. I'm talkin fast accurate bass, not boom.