Using Rew to find Crossover point?

Tough getting speaker output at that low of frequency. I'd still measure at the LP and smooth it (1/6 db per octave maybe). Just beware that you'll be getting room modes more than speaker output.

Do you know when the woofer on you CS1s start to see excursion?

Or measure outside. Or measure nearfield.

Almost every manufacturer FR curve I have seen is done at the 1W level where the drivers are hardly being stressed. FR can and does vary with levels (look at Illka's tests), esp as the LF drivers are approaching their limits both in terms of excursion and distortion. So, your 55Hz LF point might be OK at low level, but may sound stressed at higher levels using that as the xover point, whereas a say 80hz xover will give about 2/3 less excursion of the LF driver for the same level, snd lower distortion up into the midband.

Based on this, I also would aim for 80hz. There are a few reasons 80 would be better than 60. You want some extention beyond the XO point to properly blend with the sub, or you'll get asymmetric XO slopes.

That's a good idea.