I have a totalscan and a tm150, and have had a 70/200/455/800. I've had a bird chirp xducer on a helix 9g2 as well. The tm150 gave the best images regardless of depth. Cant speak to the p66 as I've never owned one. From what I've seen however chirp seems to beat fixed frequency ever time.
Getting into the science of it.. output power is only half the equation, sensitivity is the other.
Cheers,