The 1.55 and 1.56 firmware versions were essentially bulletproof to run large outrunners (2500 -3000+ watts) and not have any motor/controller sync (timing issues) a.k.a. screeching. The downside is that you lose some features that the later firmware version have. Not all set ups need to fall back to the 1.55 or 1.56 but some just wont work right with the later firmware. Some do, some don't. Even with the same make and model of motors.