Hi Kaushik,
Did you finally manage to find one? Even I am looking for a 12K range subwoofer.
Oops I almost forgot about this thread after shelving off the idea as I had to divert the funds for my other interests. And well, within 12k I didn't find anything which was worth buying. But adding just a bit more (approx. 14.5-15k) there was a option for Sonodyne 210 Roarr. I found this sub to be quite good for the price, good sound quality and worth the money. If I again have the budget then I probably will be going for this Sub. Another option is the Wharfedale SW150, read good reviews about it but didn't had time to audition it. But price was around 18-19k that time, don't know the current price for any of these actually.