I myself hunted for soft strings to use as crosses in the past. Then I tried firmer ones, and found that they worked better with the (relatively stiff) polies.
Gamma TNT2 worked very well as a cross. It better than syn guts or Isospeed Classic or Head Rip Control or a few other (accidental) strings I tried. By better I mean it game a more uniform stringbed with a good spring-back effect, and it lasted noticeably longer than all others.
Syn guts produce a stingbed that's too dead for my liking, and soft multies produce a stringbed that's nice, but inconsistent (and seems to fluctuate a lot from day-to-day).