Hi Mars,
I'm sorry to hear that MIS is not working well for you. In fact I had the exact same experience as you're having right now with MIS. 12 weeks is long enough, move on! As you have probably read on the forum, there is definitely some debate on the bulking issue. I personally think that the reason there is debate is because different people respond differently to weightlifting. I for one, like you, tend to bulk up easily. I already have a lot of muscles, so when I lift heavy they just get bigger, period. It sounds like you are already in great shape, so maybe that is what is happening to you too.
I'm currently on a PH rotation three times a week, which I find works much better for me than MIS at 2 times a week. I won't go heavier than 30 for squats and 25 for lunges, which actually might even be a little heavy, I may need to lower that to like 25 and 20.
I've really been trying to figure out what I want for my next rotation. I was considering PS, but I really think I will bulk up from it. I just flat out will not even consider S & H. I've heard alot about firm Volume 1 and I really think I might try out that video and do it three times a week for my next rotation. I guess this is kind of my theory: If you can feel it working, than it probably is. And honestly with squats and lunges I don't always feel them right there in my butt where I want to see a change. But I guarantee you with floorwork you feel it in all the right places. I think I'll try to find this tape somewhere. Someone said that you could get it at firmdirect but I didn't see it there. I know that PS also has floorwork but the workout is mainly a slower heavier format and that just turns me off because of the bulking issue. I don't care what anyone says about it, I know how my body responds to those and I don't like it!
Another issue that needs to be addressed, someone already talked about it above, is diet. Have you been watching what you eat? See the problem here though is that you already said that your body fat is fairly low. What was it again? So maybe your diet may be a factor and maybe not.
Let me know what you do. I am either going to buy vol 1 (if I can find it) or one or two of those other tapes mentioned above for my next rotation. If I can't find volume 1 then I'll just buy a floorwork tape and an upper body tape somewhere.
Another possibility is that you could also try Leaner Legs. It is an endurance workout and supposedly a killer. I don't have it yet but am definitely going to buy it soon.
Sorry this is sooo long.
Good luck!
Kristie