CGB: You doubt an advanced trainee can add 10-15 lbs without supplements?
I can. I followed GVT and John Berardi's diet recommendations, without any supplements with the exception of protein powder for post workout, and I gained 14 lbs. I've been training for 4 years.
10-15 lbs is ABSOLUTELY accomplishable in 12 weeks. I put on 14 lbs in 8 weeks. Considering your hormone profile is decent (meaning your testosterone levels are average), you get decent sleep, your diet is finely tweaked as per JB, you follow a mass gain routine balls to the walls, and your stress levels arent out of line...then, YES YOU CAN gain 10-15 lbs in that period of time VERY easily.
Now, if everytime you gain one lb you go reduce your calories because you're afraid to lose your abs, you're not gonna accomplish anything. If you smash your balls to the wall at the gym and eat right and enough calories to support new growth, you will grow.