Actually it's a personal issue I'm having while following CGtuts+ tutorial for modeling a cool speaker for the design of my musician friend's web site. I posted the problem I'm having to modo forum on cgtalk and got some cool response
Mentor is not subdividing the mesh on the tutorial. But when I tried I had that problem because of the n-gon surfaces I think.
"If you still want to subdivide it and do it properly to get those nice edges you should add more geometry (edges) which will "hold" the circular shapes." - Nino
And here is how we should split the geometry to hold the circular shapes.
And here's the subdivision result.
Hope it will be useful for the people who having the same issues. And here's Glen Southern's Sub Division Modeling web site