Adding 2nd subwoofer to a 12 inch subwoofer

Hi, I have a diy 12 inch sony xplod subwoofer which is quite good for my 14ft x 14ft room. I have a creative x-fi sound card and I run 7.1 speaker with CMSS and audio crystalizer on.

For a more cleaner range of bass I want to add a second subwoofer. Should I go for a 10 inch or another 12 inch subwoofer?
 
Okay, seems you may be under the impression that bigger drivers are struggling with reproducing the higher bass frequencies.
That is pure BS, sorry. Even an 18" has no problem going up to 4-500hz, if you hear people talk about slow and fast bass it is more about the implementation than the size of the driver.

Graham is correct. Also often better to duplicate the sub you have, use some random microphone to try and measure what you have. Try and get phase and frequency issues worked out by placement and/or eq/dsp. If you got ported speakers try to plug the ports to make it a bit easier to cross over to the sub.
 
Right, it will be pistonic to the VC diameter, so even a 5" VC can do a usable ~862 Hz if not cone designed and/or doped to roll off lower, though whether one wants to from a sonic POV is another matter.

GM