You can easily combine an active sub with the KRK5's - you just wire the audio first to the subs, then from the subs to the mains. The sub takes care of "splitting" the signal between the sub and the mains. IMHO sub selection doesn't matter nearly as much as the main monitors. I happen to have a...
I would think that the specification actually reads "Frequency Response (Boundary Position): 28-170 Hz, rather than "Boundary Position: 28-170 Hz", which makes pretty much no sense. If so, it's the frequency response at the room boundary. Probably a -6dB reading.
My experience with a 64F8500 is that movie setting is very close to a good calibration; mine just needed some fine tuning of the brightness and contrast
You want 75 Ohm Coax - commonly available are RG59 and RG6. RG6 has lower attenuation, so will be better over long distances. But unless you're running over more than 10's of meters, it won't make a difference.