Lowest Freq for a 2 looneys?

You shouldn't be setting your HPF any lower than 42-45Hz.

According to hornresp i would say down to 43hz group of 4.
So with -3dB point @45hz your guess is pretty well, but for 4.
For a pair its not much higher.

I made the simulations with a void1000

keep in mind beyond the horncutoff the "horn loading" and so your "throw"
is becoming uneffective and your speaker works as a closed box,
but with a awful high xmax that destroys your speaker with enough power easily.