New Irons, what shaft should I get?

I currently have project x 5.5 shafts in my irons. These are not stiff enough and I hit the ball rather high. I am getting the MBs this winter but don't know which shaft to order. My average swing speed with a driver is a little over 120 mph. Any suggestions as to which shaft I should order? 

Tiger, I glad your getting new MBs and discarding that OTHER brand.. I wouldn't go to KBS if you need to flight down.  I suggest S300/400 or PX 6.0/6.5..maybe 7.0?

Just from the info you have listed it sounds like you maybe a good candidate for the KBS C- Taper shaft, maybe S+?  Everyone's swing has their own dna, Tempo, Speed, Load & release you just can't be fitted correctly thru limited information. Their are too many great shaft options out today for all different types of swings, what works well for 1 player may perform terrible for another with similar speed.

Go see a Titleist fitter, that's what the fitting carts are for!