Box and port are too small, period. Even though LMS like small boxes they don't like small ports. That's a big cone, strong motor and high xmax subwoofer, it wants a BIG port, regardless of the box size you put it in. Port area/cube rules of thumb don't work with powerful woofers. You could realistically get away with 5 cubic feet net if you went with plenty of port area. You simply don't have the room to run this thing ported.. Your only realistic option is sealed or passive radiators. If you can fit 3.5 net you probably have close to 5 gross, making passives an attractive option, probably your best bet if you want ported output in a small box.
You can ignore everyone here if you'd like, but your just making a bad box for a nice sub.. If you want mediocre results, that's on you. A 3x17inch port WILL chuff at high volume levels.. That's basically a port size for a 12 or 15 inch woofer.. The reason the LMS is good in a small box is because it has a low fs and a very strong motor. That strong motor is also very good at exciting a port even in a small box, especially if you tune low, which is probably part why you picked an LMS, for low end output. Go ahead choke that bish and listen to port noise and distortion, good use of $800..
edit: The bassbox pro reccomendations youru trying to use are crap, as are most default BB6 alignments for powerful subwoofers.. My 21 has a very powerful motor, as strong as the LMS 5200, even stronger than the Ultras. Bass prox pro high fidelity alignment is 2.22 cubic feet.. Let's see 14inches/cube means my 21inch subwoofer with a 6inch voicecoil and 4k powerhandling needs a 2.25 cube box and 28square inches of port area... Guess I overdid it with a 6x22.5inch port huh? //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif
http://www.parts-express.com/bc-21sw152-4-21-neodymium-subwoofer-4-ohm--294-689
