Looking for a soft arm friendly poly? Check out Weiss Cannon Scorpion. I've been playing with this string for the past few months. On my second reel now and still loving it. It has good power and it is a SOFT poly. Personally, it feels even softer than SPPP.
Right out of the package you can tell it's more flexible than a string like Tecnifibre Pro Red Code. I've been using it in a stiff frame POG Mid and haven't experienced any pain yet. Also, this string gets pretty good reviews for tension maintenance and it has decent spin. It's no ALU Rough, but you can still generate plenty of spin.
If you seek comfort, this string is definitely worth a try.