I got a surround repaired on one of my PRs-an 18inch, it was done in a local PA speaker shop and cost me 750. The best bet is to contact a public address equipment outfit they use these for PA systems.
If the surround is damaged it is better that you stop using the speaker.Otherwise you are likley to damage the voice coil,due to the non linear movement of the cone.
There are several speaker repair shops on/around Ritche St. the best of them is Ali of Samra Electronics . His shop is on the second floor in the building next to Torvin. He does all the speaker repair work for The Audio People.The problem with him is that he is erratic with delivery.
Another option is Patwal Electronics on the corner of Ritche St. opp the juice shop.
It should not cost more than Rs.1000/-most probabaly less.
I suggest that you take your driver over to Electronic Service Co.the Philips Arena showroom on Mount Road(opp Spencers) and ask Mr.Sayeed(he is Ali's uncle) to get the surrounds replaced.This is your best option for a stress free repair,but it may be slightly more expensive.
I am surprised why you must run around in Chennai for your speaker repair. Pl. contact your DrfTech Dealer from whom you purchased your SW for professional Quality repair. However if u hav purchased your subwoofer in Grey market or if you hav imported ,then you hav to try local repair shops around Riche street and choose the best among them.