Since you used the RPM Blast you should try its softer version RPM TEAM its injected with micro air bubbles which makes it softer. I currently use these strings and love them! its got great durability too!
There are only a few soft polys out there and most of them are stiff. If you really want to soften out a poly i recommend using a hybrid with either Natural gut (if you have the money), synthetic gut (i like the x-one biphase from tecnifibre), and or a multifilament.
Babolat Pure Drive 2012 | Gamma TNT2 @ 61 lbs