The P66 is a good transducer but it can't do CHIRP. The Gen-2 is a 500 watts units and the Gen-3 is 1000 watts unit. What I would do is to use the Gen-3 in split screen using the P66; 1 part of the screen on 50kHz and the other on 200 and I'll be using the Gen-2 for mapping only.
If you fish LO I will use Fresh setting, low noise is OK, for color palet I mainly use # 1 and sometimes # 13. For sensitivity : on auto but raise it until you see a little bit of "rain". Also another important thing is to be sure that the right transducer is choose under settings-Sonar-Installation and scroll until you can choose Airmar P66; there is 2 choices , 1 with a 5 and the other with a 10 at the end; there is one that work with the temp and I think it's the 10. Colorline is something useful specially to identify the bottom hardness.