Tell us about yourself!
Also - are your dimensions in your diagram removing the MDF thickness? For example is your usable port area 3in x 16.5in as stated or 1.5in x 15in with the mdf thickness taken out?
Can you let me know how you derived your chamber volumes and port tuning? I also have a Sundown X12D2 rev2 and am thinking of experimenting going from a ported to 4th order bandpass enclosure. However using WinISD with Sundown's posted T/S parameters I can't match your numbers.In the attached images I show the ported chamber at 3 cu ft tuned to 47hz and sealed chamber at 1.34 cu ft. The port length for a 3in x 16.5in area it calculates at 12.6 in long vs your 8 in port length. WinISD's gain plot also looks pretty ugly, one very narrow peak. Obviously I'm having problems :)