If you're going to end up needing a sub anyway, I'd strongly recommend getting a servo controlled OB sub. Here's a link to one that's DIY that will be the last sub you will ever need. Fully OB and will punch you in the chest like a Mofo, goes all the way down to 16hz strong and goes LOUD:http://gr-research.com/servosubkit4.aspx
I use a pair of them, built into my Super 7 speakers, and trust me this sub will rock your world.
Oh, but compared to what? Well I've been a bass head all my life, I've owned and built some crazy system. The latest thing I've built is this box sub from Dayton Audio, the 18" Ultimax:
I'll tell you what, that 18" Ultimax with a Crown Audio K2 doing 1000 watts into it is freaking awesome. How does the GR Research OB setup compare? The GR Research OB setup is so much better, I'd take it over the Ultimax every day of the week. And twice on Sundays.
Combined with the M3 you'd have a reference level system that you could live with for a very long time, IMO.