hi there i dont know if you remember you designed a box for me a 3.3ft3 after disp tuned to 30 hz.
and might i add it sounded awesome but my problem is it was bottoming out too quick or reaching its xmax too quick.

wat i would like is another box designing like the last one but with the sizes massive have sent me .but from wat i can see it dont tell me much but ill give you the mail i got if thats all right with you.

The total internal volume of the enclosure for CW 12d should be approximately 1.8 ft3 with 4" X 10" port.
You can use a bigger box with no problem, it will be more sensitive good for sound competition @ one frequency
but you may have problem with music or frequencies lower than the port tuning.( depending to your box & tuning)
In general, the impedance of the large boxes will be lower only at port frequency but will increase sharply
at any frequency below the tuning. This may cause the voice coil to produce heat instate of movement.

The other possibility is that the sub is bottoming out. CW 12d has 13mm X-max which is very good.
With correct tuning of the box you will never have x-max problem.
My suggestion is to take out the sub, check the impedance of each voice coil with a voltmeter, they should be 4 Ohms
then you can play the sub @ low volume free air and see it you get any distortions.

If there is not problem with the sub, you should redesign your box.

i hope this is some help m8 and heres the sub peramitors again,

Standard Metric
Fs 32.84 Hz 32.84 Hz
Qts 0.368 0.368
Vas 1.42 ft3 40.01 l
Impedance 4 + 4 4 + 4
Xmax 0.591 " 15.0 mm
Winding Length 1.250" 31.82 mm
Recommended Power 600W 600W
SPL 87.79 dB 87.79 dB
SPL (2.83-V @ 1m ) 94.04 dB 94.04 dB
Displacement 0.095 ft3 2.70 l
Frame Diameter (A) 12.25 " 31.12 cm
Mounting Hole Diameter (B) 11.25 28.58 cm
Mounting Depth (C) 6.25 "15.88 cm
Overall Depth (D) 7.25 18.42 cm
Printed in U.S.A.

i hope this is off some help and you dont mind hepling me out friend.

oh and i like sql about 28 hz or there abouts