I just recently purchased a Mesa Powerhouse 410 (8ohm)
and would like to power it with my GK 700rbII. Should I use the GK speakon cables I have (used to run SBX410 cabs) or would 1/4" phono cables be better?
No problem to use the GK cable, understanding that you wont have separate volume control for the mesa tweeter. You will be operating full range, and only the "woofer" gain control on the GK amp will function. But the combination will sound fine - I know, because I have the same.
If the 2 high connections aren't connected - will the horn be off then - not working at all?
The horn will work as usual. People were talking about the biamp system in the 700RB-II, but your Mesa 4x10 will work just as intended with full range input.