I saw this interview of Balu only recently. As usual he was very simple and modest.

Here is a bit of it.

Anuradha SenGupta: When you sing for Rajinikanth, or Kamal Hassan, I don’t think you are trying to sound like they are singing. Am I right?

SP Balasubrahmanyam: Most people think that I change my voice for actors that I sing for. But that is not right. You are absolutely correct. The style can be different. I just try to get the vibrancy of actors in my singing. When I sing a romantic number for Kamal, I try to get the style in my singing. I mostly sung for them. It also helped me to dub them properly.

My take on this: First of all, I feel that that was a very bad question asked by the interviewer. She should probably have asked him if he had tried to sound like them rather than "telling" him what she thought. I don't know well how he sang for Kamal or Rajini(as I don't know much about Tamil songs), but there are many instances where he sounded like the actor himself singing. He is being modest as usual.

You can watch/read the interview here.
http://ibnlive.in.com/news/being-balasubrahmanyam-the-perfectionist/68767-8.html