It seems that the S300 and the pro 105 have the same flight characteristics (low)....I'm not sure if the flex point or the stiffness is the same but , it does sound like a shaft problem...I use the AP2's with s300 and use a 9 iron for 155 so the distance is there with the s300...maybe try the AP2's with the s300 to see for yourself if the distance is there....Good luck...

I really like the AP2 as well just bought my set and i had a demo day and still was a very difficult decision between two shafts i ended up getting the kbs tour and loved them almost went with the kbs c-taper for lower ball flight which is what you might need to get your yardage back i really enjoyed both but for me as a beginner i thought a little higher trajectory might help me. i recommend trying the c-tapers hope it helps

It may just be something in your swing.  You should do a side by side comparison of your old irons and your new ones and then go from there.